<PAGE>

                                                                   Exhibit 10.50

                               SECURITY AGREEMENT

This Security Agreement ("Agreement") is made and entered into as of December
___, 2001, by and between U.S. XPRESS LEASING, INC., a Tennessee corporation
("Debtor"), and DAIMLERCHRYSLER SERVICES NORTH AMERICA LLC, 1011 Warrenville
Road, Lisle, Illinois 60532 ("Secured Party").

                        Article One - SECURITY INTEREST

For value received, Debtor grants to Secured Party a security interest in all of
Debtor's right, title, and interest on, to and under the following property and
interests in property of Debtor, whether presently in existence or whether now
owned or hereafter arising, acquired or created by Debtor at any time hereafter,
wherever located, and any replacements, additions, accessions or substitutions
thereto (collectively, the "Collateral"):

      1.1    the eight hundred eighty-six (886) tractors of Borrower described
             on Exhibit A attached hereto and made a part hereof (collectively
                ---------
             referred to as the "Equipment" and individually referred to as a
             "Unit of Equipment"), together with;

      1.2    all proceeds and products of any and all of the foregoing,
             including but not limited to all chattel paper, electronic chattel
             paper, goods (including, without limitation, motor vehicles,
             tractors, trailers, chassis and equipment taken in trade), contract
             rights, leases, accounts, documents, instruments, promissory notes,
             general intangibles, payment intangibles, supporting obligations,
             claims and tort recoveries, money, insurance proceeds and refunds
             of insurance premiums, in each case, relating to or arising out of
             any of the types of collateral described in Section 1.1 above, and
             all proceeds of such proceeds and products; and

      1.3    all books, records, computer records, computer disks, ledger cards,
             programs and other computer materials, customer and supplier lists,
             invoices, orders and other property and general intangibles at any
             time evidencing or relating to any of the Collateral.

The definitions of the types of Collateral described in paragraph 1.2 are
intended to expand as the definitions and descriptions of such types of
Collateral that are set forth in the Code expand. The term "Code" means the
Uniform Commercial Code, as enacted in the State of Tennessee, as amended and
replaced from time to time.

To    secure: (1) Debtor's Promissory Note to the Secured Party, of even date
      herewith, in the principal amount of $53,310,901.58 (as the same may from
      time to time be amended, modified, extended, renewed or restated, the
      "Note"), principal and interest payable as provided in said Note;

<PAGE>

(2)   the Loan (as defined in the Loan Agreement) and all obligations of Debtor
      to Secured Party under the Loan Agreement, of even date herewith, between
      the parties (as the same may from time to time be amended, modified,
      extended, renewed or restated, the "Loan Agreement");

(3)   all expenditures by Secured Party for taxes, insurance, repairs to, and
      maintenance and preservation of the Collateral, and all costs and expenses
      incurred by Secured Party in the collection and enforcement of the Note,
      the Loan and the other Obligations (as defined in the Loan Agreement) or
      in representation of Secured Party in connection with bankruptcy or
      insolvency proceedings, including, without limitation, legal fees and
      expenses of both in-house and outside counsel; and

(4)   all of the Obligations (as defined in the Loan Agreement) of Debtor to
      Secured Party now existing or incurred in the future, matured or
      unmatured, direct or contingent, and any renewals and extensions of, and
      substitutions for such Obligations.

The security rights and interests of Secured Party in the Collateral will
continue until the Loan and all of the other Obligations, including possible
contingent Obligations, are fully paid and satisfied and Secured Party elects to
cancel and terminate its security interests in writing. This is a continuing
security agreement, which will continue in effect until canceled by Secured
Party even though all or any part of the Loan and the other Obligations may be
paid in full, and even though for a period of time Debtor may not be then
obligated to Secured Party.

                  Article Two - USE AND LOCATION OF COLLATERAL

Debtor warrants and covenants that:

      2.1    The Collateral is to be used in Debtor's business as a truck
             transportation company and shall not be used for personal, family,
             household or agricultural uses or purposes.

      2.2    Debtor's headquarters and principal place of business is located at
             4080 Jenkins Road, Chattanooga, TN 37421.

      2.3    The Collateral will be kept in Debtor's control and possession
             (except in accordance with and subject to the terms and conditions
             of Section 2.4) and will be used only in the continental United
             States of America, Canada, and Mexico.

      2.4    Debtor shall be permitted to enter into lease agreements covering
             Items of Equipment in the ordinary course of its business, provided
             that the following conditions are met (such leases are referred to
             as "Permitted Leases"): (a) all leases must be in writing using the
             term of lease delivered by Debtor to Secured Party, (b) the term of
             each lease shall not extend beyond the maturity date of the Note,
             and (c) the number of Units of Equipment subject to leases shall
             not exceed the lesser of (i) eighty (80) Units of Equipment and
             (ii) ten percent (10%) of the total number of Units of Equipment
             then subject to this Security Agreement in

                                      -2-

<PAGE>

             each case reduced by the number of vehicles financed by Secured
             Party after the date hereof which are leased.

             Article Three - TITLE TO AND MAINTENANCE OF COLLATERAL

Debtor warrants, covenants, and agrees as follows:

      3.1    Title. Except for the security interest granted by this Agreement,
             Debtor has full title to, rights in and the power to transfer the
             Collateral free from any lien, security interest, encumbrance,
             lease, other than those listed in Schedule 3.1 hereto and leases
             permitted by Section 2.4, restriction, pledge, or transfer or other
             claim and the Debtor will, at Debtor's cost and expense, defend any
             action that may affect Secured Party's security interest in, or
             Debtor's title to, the Collateral.

      3.2    Perfection of Security Interests. Secured Party may file whatever
             financing and continuation statements, amendments, and other
             documents, and may take whatever additional actions, it deems to be
             necessary and proper to perfect and continue the perfection of
             Secured Party's security interests in the Collateral. To the extent
             that Secured Party may have previously filed financing statements
             affecting any of the Collateral, Debtor ratifies and confirms
             Secured Party's authority to do so and the contents and binding
             effectiveness of such financing statements. Secured Party may file
             a carbon, photographic, facsimile, other reproduction or
             electronically authenticated or maintained copy of any financing
             statement or of this Agreement for use as a financing statement.
             Secured Party may make electronic filings of financing and other
             statements. All filings under this Section, including, without
             limitation, electronic filings, will be deemed to be complete and
             perfected for all purposes when made by Secured Party and may be
             made by Secured Party without the necessity that Debtor (or Secured
             Party on Debtor's behalf) sign any such financing statements or
             other perfection documents. Debtor shall reimburse Secured Party
             for all expenses incurred with respect to the perfection and
             continuation of the perfection of its security interests in the
             Collateral. Without limitation of the generality of the foregoing:
             (a) to the extent that any of the Collateral is held by a third
             party (such as consignee or bailee) (i) notice of the security
             interests created by this Agreement in such Collateral shall be
             given to each such third party and (ii) Debtor shall, upon the
             request of Secured Party, obtain and deliver to Secured Party a
             written and signed acknowledgment from each such third party that
             it is holding the Collateral for the benefit of Secured Party; (b)
             to the extent that any of the Collateral is comprised of electronic
             chattel paper, Debtor will ensure that (i) there is only one
             identifiable authoritative copy of the electronic chattel paper
             record, (ii) the authoritative electronic chattel paper record for
             all electronic chattel paper in which Secured Party has a security
             interest will identify Secured Party as the first lien-holder,
             (iii) the authoritative electronic chattel paper record for all
             electronic chattel paper in which Secured Party has a security
             interest will be transferred to and maintained by Secured Party or
             a third party custodian designated by Secured Party and (iv)
             changes or additions to the electronic chattel paper may not be
             made without the consent of Secured Party; (c) to the extent that
             any of the

                                      -3-

<PAGE>

             Collateral is comprised of types of Collateral that can be
             perfected by possession or by either possession or filing, all such
             Collateral shall be delivered to Secured Party; and (d) Debtor
             agrees to execute any further documents, and to take any further
             actions, reasonably requested by Secured Party to evidence, perfect
             or protect the security interests granted herein or to effectuate
             the rights granted to Secured Party herein. Secured Party is also
             authorized, at Debtor's cost and expense, to obtain all post-filing
             searches from all jurisdictions that Secured Party deems advisable
             to confirm the proper priority of all filings made by Secured Party
             under this Agreement. Debtor appoints Secured Party its true and
             lawful attorney-in-fact, coupled with an interest, for Debtor and
             in its name, to execute and sign on Debtor's behalf, and to take
             all acts on Debtor's behalf, that are required under this Section
             or that may otherwise be required to perfect, maintain and protect
             Secured Party's security interests in the Collateral, including the
             execution of financing statements and other documents on Debtor's
             behalf. Such power of attorney may not be revoked.

      3.3    Titled Collateral. A certificate of title or similar titling or
             registration document has been issued by the state of Tennessee for
             each item of Equipment. Such Equipment shall be titled/registered
             in the name of Debtor and Secured Party shall be shown as the first
             priority and only lienholder on the certificate of
             title/registration for each such item of Equipment. A copy of the
             certificate of origin, application for certificate of title or
             registration and/or any other forms required to satisfy the
             foregoing titling/registration requirements and to perfect Secured
             Party's first priority security interests in such Collateral shall
             be delivered to Secured Party, (a) immediately and (b) immediately
             upon Debtor's acquisition of any such Collateral in the future. In
             addition, a copy or the original, as required by Secured Party, of
             the issued certificate of title/registration for each such item of
             Collateral shall be delivered to Secured Party as soon as it is
             issued. Secured Party may retain physical possession of those
             certificates. Debtor shall not change the state in which any of the
             Equipment covered by this Section is titled/registered to other
             than the state of Tennessee without the prior written consent of
             Secured Party.

      3.4    Sale, Lease or Disposition of Collateral. Except for Permitted
             Leases, Secured Party does not authorize, and Debtor will not,
             without the prior written consent of Secured Party, sell, contract
             to sell, lease, license, encumber, grant any security interest in
             or dispose of the Collateral or any interest in it until this
             Agreement, the Loan and all of the other Obligations have been
             fully satisfied; provided, however, that Debtor may sell, convert
             or write down its inventory, in the ordinary course of business.

      3.5    Insurance. Debtor will insure the Collateral on a replacement cost
             basis with companies acceptable to Secured Party against the
             casualties and in the amounts that Secured Party shall reasonably
             require with a loss payable clause in favor of Secured Party.
             Without limiting the foregoing, Debtor shall continuously maintain
             physical damage insurance with Secured Party as sole loss payee
             with a deductible of not more than $5,000 per item of Equipment and
             liability insurance

                                      -4-

<PAGE>

             with Secured Party as additional insured with a combined single
             limit coverage of not less than $750,000. Unless a Default shall
             have occurred and be continuing, Debtor shall be permitted to
             collect the proceeds of insurance and to use such proceeds to
             repair the item of Equipment with respect to which the proceeds are
             received or, with the prior consent of Secured Party, to obtain a
             replacement item of Equipment. In all other cases, Secured Party is
             authorized to collect proceeds from any of the insurance policies
             and apply them to the Obligations, or to allow repairs or purchase
             of a replacement item of Equipment at its sole discretion.

      3.6    Assignment of Insurance. Debtor hereby assigns to the Lender, as
             additional security for payment of the Note and other Obligations,
             any and all moneys due or to become due under, and all other rights
             of Debtor with respect to, any and all policies of insurance
             covering the Collateral. Debtor hereby directs the issuer of any
             such policy to pay any such moneys directly to Lender following the
             occurrence and during the continuance of a Default, during which
             Lender may (but need not), in its own name or in Debtor's name,
             execute and deliver proofs of claim, receive such moneys, endorse
             checks and other instruments representing such moneys and settle or
             litigate any claim against the issuer of any such policy.

      3.7    Protection of Collateral. Debtor will keep the Collateral in good
             order and repair and will not waste or destroy the Collateral or
             any part thereof. Debtor will not use the Collateral in violation
             of any statute or ordinance and Secured Party will have the right
             to examine and inspect the Collateral at any reasonable time.

      3.8    Taxes. Debtor will pay promptly when due all fees, costs, taxes,
             assessments or expenses on or in connection with the Collateral
             and/or for its use, operation and titling.

                     Article Four - PROTECTION OF SECURITY

      4.1    Attorney in Fact. Debtor hereby appoints and constitutes Secured
             Party and its agents and designees, as Debtor's attorney-in-fact,
             at Debtor's own cost and expense, to exercise at any time following
             the occurrence and during the continuation of a Default under the
             Loan Agreement all or any of the following powers, which, being
             coupled with an interest, shall be irrevocable until all of the
             Obligations have been paid in full: to receive, take, endorse,
             assign, deliver, accept and deposit, in the name of Secured Party
             and/or Debtor, any and all checks, notes, remittances, drafts and
             other documents and instruments and documents relating to the
             Collateral; to receive, open and dispose of all mail addressed to
             Debtor relating to the Collateral and to notify postal authorities
             to change the address for delivery of mail to such address as
             Secured Party may designate; and to take or bring, in Secured
             Party's and/or Debtor's name, all steps, actions or proceedings
             deemed by Secured Party to be necessary or desirable to effect
             collection of the Collateral or to preserve, protect or enforce
             Secured Party's interests therein. Said attorney, agent or designee
             shall not be liable for any acts or omissions, nor for any error of
             judgment or mistake of fact or law.

                                      -5-

<PAGE>

     4.2  No Liability of Secured Party. Nothing herein contained shall be
          construed to constitute Debtor as Secured Party's agent for any
          purpose whatsoever. Secured Party shall not be responsible nor liable
          for any shortage, discrepancy, damage, loss or destruction of any
          Collateral wherever the same may be located and regardless of the
          cause thereof. Secured Party shall not, under any circumstances, have
          any liability for any error or omission or delay of any kind occurring
          in the settlement, collection or payment of the Collateral or any
          instrument received in payment thereof or for any damage resulting
          therefrom. Secured Party may, without notice to or consent from
          Debtor, sue upon or otherwise collect, extend the time of payment of,
          or compromise or settle for cash, credit or otherwise upon any terms,
          any of the Collateral or any securities, instruments or insurance
          applicable thereto and release any obligor thereon. Secured Party
          shall not, by anything herein or in any assignment or otherwise,
          assume any of Debtor's obligations under any contract or agreement
          assigned to Secured Party, and Secured Party shall not be responsible
          in any way for the performance by Debtor of any of the terms and
          conditions thereof.

     4.3  Debtor Responsible for Expenses. Any fees, costs and expenses, of
          whatever kind and nature, including taxes of any kind, which Secured
          Party may incur in filing public notices, as well as expenses incurred
          by Secured Party (including all attorneys' fees of both in-house and
          outside counsel), in protecting, maintaining, preserving or enforcing
          the Obligations, the Collateral or the pledges, liens and security
          interests granted to Secured Party hereunder, whether through judicial
          proceedings or otherwise, or in defending or prosecuting any actions
          or proceedings arising out of or related to any of the Obligations or
          Secured Party's transactions with Debtor, including actions or
          proceedings which may involve any person asserting a priority or claim
          with respect to the Collateral, shall be paid for by Debtor on demand
          and until paid by Debtor shall be added to and deemed part of the
          Obligations.

     4.4  Decrease in Value of Collateral. If in Secured Party's reasonable
          judgment the Collateral has materially decreased in value or if
          Secured Party shall deem that the Collateral at any time is not of
          sufficient value to secure fully the total Obligations of Debtor to
          Secured Party, Debtor shall, within ten (10) days following notice
          from Secured Party, either provide enough additional Collateral or
          reduce the total Obligations by a sufficient amount to satisfy Secured
          Party that its security is adequate.

     4.5  Reimbursement of Expenses. At its option, Secured Party may discharge
          taxes, liens and interest, may perform or cause to be performed for
          and on behalf of Debtor any actions and conditions, obligations, or
          covenants that Debtor has failed or refused to perform, and may pay
          for the repair, maintenance and preservation of the Collateral, and
          all sums so expended, including but not limited to attorney's fees (of
          both in-house and outside counsel), court costs, insurance premiums,
          agent's fees, or commissions, or any other costs or expenses, shall
          bear interest from the date of payment at the annual rate of eighteen
          percent

                                      -6-

<PAGE>

          (18.00%) or, if lower, the maximum lawful rate and shall be payable at
          the place designated in the Note and shall be secured by this
          Agreement.

                        Article Five - DUTIES OF DEBTOR

     5.1  Payment. Debtor will pay and/or perform the Obligations in accordance
          with their terms and will repay immediately all sums expended by
          Secured Party in accordance with the terms and provisions of this
          Agreement.

     5.2  Change of Place of Business. Debtor will promptly notify Secured Party
          of any change of Debtor's places of business, or places where records
          concerning the Collateral are kept.

     5.3  Waiver. Secured Party's acceptance of partial or delinquent payments,
          or the failure of Secured Party to exercise any right or remedy, shall
          not be a waiver of any obligation of Debtor or right of Secured Party
          or any other similar default subsequently occurring.

                       Article Six - DEFAULT AND REMEDIES

     6.1  Default. Debtor shall be in "Default" under this Agreement on the
          occurrence of any Default under Article 6 of the Loan Agreement.

     6.2  Remedies. Upon the occurrence of any Default, Secured Party shall have
          all of the legal remedies set forth in Article 7 of the Loan Agreement
          and all other remedies created or existing under applicable law, under
          the Code, in equity, under the Loan Documents or under any other
          agreement between Debtor and Secured Party or from Debtor to Secured
          Party.

                         Article Seven - MISCELLANEOUS

     7.1  Time of Essence. Time is of the essence in this Agreement.

     7.2  Tennessee Law to Apply. This Agreement shall be governed by the Code
          and other applicable laws of the State of Tennessee, and the parties
          hereby submit to the jurisdiction of the courts of that State;
          provided, however, that Secured Party shall have the right, but not
          the obligation, to litigate in any state or country in which Debtor,
          any Guarantor, or any of their assets may be located.

     7.3  Parties Bound. This Agreement shall be binding on and inure to the
          benefit of the parties hereto and their respective successors and
          assigns. Notwithstanding the foregoing, Debtor may not assign its
          rights or duties hereunder without the prior written consent of
          Secured Party.

     7.4  Notices. All notices and payments shall be mailed to the respective
          addresses of the parties at the addresses set forth at the beginning
          of this Agreement, or such other address as either party may provide
          to the other from time to time in

                                      -7-

<PAGE>

          writing. Notices sent to Secured Party shall be sent to the attention
          of Robert D. Konneker.

     7.5  Legal Construction. In case any one or more of the provisions
          contained in this Agreement shall for any reason be held invalid,
          illegal or unenforceable in any respect, such invalidity, illegality
          or unenforceability shall not affect any other provision of this
          Agreement and this Agreement shall be construed as if such invalid,
          illegal or unenforceable provision had never been contained herein.

     7.6  Prior Agreements Superseded; Changes in Writing. This Agreement may
          not be changed, waived, discharged or terminated without an instrument
          in writing signed by the party against whom the change, waiver,
          discharge or termination is sought.

                                      -8-

<PAGE>

         IN WITNESS WHEREOF, the parties have caused this Agreement to be
executed by their duly authorized representatives as of the date first above
written.

                            DEBTOR:

                            U.S. XPRESS LEASING, INC.

                            By:
                               -----------------------------------------------
                                                Signature
                            Title:____________________________________________


                            SECURED PARTY:

                            DAIMLERCHRYSLER SERVICES NORTH AMERICA LLC

                            By:
                               -----------------------------------------------
                                                Signature
                            Title:____________________________________________


                                       -9-

<PAGE>

                                  SCHEDULE 3.1
                                  ------------
                                       TO
                            U.S. XPRESS LEASING, INC.
                               SECURITY AGREEMENT

Liens on the vehicles in favor of Wachovia Bank, N.A. which will be released in
connection with this transaction.