<PAGE>
                                                                   Exhibit 10.21


                 FOURTH AMENDMENT TO RECEIVABLES SALE AGREEMENT

THIS FOURTH AMENDMENT (the "Amendment"), dated as of December 26, 2002, is
entered into among Swift Receivables Corporation (the "Seller"), Swift
Transportation Corporation (the "Collection Agent"), Amsterdam Funding
Corporation, a Delaware corporation ("Amsterdam"), ABN AMRO Bank N.V., as
Amsterdam's program letter of credit provider (the "Enhancer"), the Liquidity
Provider listed on the signature page hereof (the "Liquidity Provider") and ABN
AMRO Bank N.V., as agent for Amsterdam, the Enhancer and the Liquidity Provider
(the "Agent");


                                   WITNESSETH:

WHEREAS, the Seller, Collection Agent, Amsterdam, Enhancer, Liquidity Provider
and Agent have heretofore executed and delivered a Receivables Sale Agreement
dated as of December 30, 1999 (as amended, supplemented or otherwise modified
through the date hereof, the "Sale Agreement"); and

WHEREAS, the parties hereto desire to amend the Sale Agreement as provided
herein;

NOW, THEREFORE, for good and valuable consideration, the receipt and adequacy of
which are hereby acknowledged, the parties hereto hereby agree that the Sale
Agreement shall be and is hereby amended as follows:

      Section 1. The defined term "Aggregate Commitment" appearing in Schedule I
      to the Sale Agreement is hereby amended in its entirety and as so amended
      shall read as follows:

      "Aggregate Commitment" means $204,000,000, as such amount may be reduced
      pursuant to Section 1.6.

      Section 2. The defined term "Liquidity Termination Date" appearing in
      Schedule I to the Sale Agreement is hereby amended by deleting the date
      "December 26, 2002" appearing in clause (d) thereof and inserting in its
      place the date "December 24, 2003".

      Section 3. The defined term "Purchase Limit" appearing in Schedule I to
      the Sale Agreement is hereby amended in its entirety and as so amended
      shall read as follows:

      "Purchase Limit" means $200,000,000.

      Section 4. The defined term "Termination Date" appearing in Schedule I to
      the Sale Agreement is hereby amended by deleting the date "December 26,
      2002" appearing in clause (c)(ii) thereof and inserting in its place the
      date "December 24, 2003".

      Section 5. Schedule II to the Sale Agreement is hereby amended in its
      entirety and as so amended shall read as set forth as Schedule II to this
      Amendment.

      Section 6. The Seller agrees to pay to the Agent for its own account a
      $50,000 amendment fee on the Settlement Date occurring in January, 2003.

      Section 7. This Amendment shall become effective on December 26, 2002 if
      the Agent has received (i) counterparts hereof executed by the Seller,
      Collection Agent, each Purchaser and the Agent, (ii) the acknowledgment
      and consent in the form set forth below duly executed and delivered by the
      Swift Transportation Co., Inc., (iii) counterparts executed by the Seller,
      the Agent, and Amsterdam to the Second Amendment to Fee Letter of even
      date herewith and (iv) counterparts executed by the Seller, the Agent and
      Amsterdam to the First Amendment to Pricing Letter of even date herewith.
<PAGE>
      Section 8.1. To induce the Agent and the Purchasers to enter into this
      Amendment, the Seller and Collection Agent represent and warrant to the
      Agent and the Purchasers that: (a) the representations and warranties
      contained in the Transaction Documents, are true and correct in all
      material respects as of the date hereof with the same effect as though
      made on the date hereof (it being understood and agreed that any
      representation or warranty which by its terms is made as of a specified
      date shall be required to be true and correct in all material respects
      only as of such specified date); (b) no Potential Termination Event
      exists; (c) this Amendment has been duly authorized by all necessary
      corporate proceedings and duly executed and delivered by each of the
      Seller and the Collection Agent, and the Sale Agreement, as amended by
      this Amendment, and each of the other Transaction Documents are the legal,
      valid and binding obligations of the Seller and the Collection Agent,
      enforceable against the Seller and the Collection Agent in accordance with
      their respective terms, except as enforceability may be limited by
      bankruptcy, insolvency or other similar laws of general application
      affecting the enforcement of creditors' rights or by general principles of
      equity; and (d) no consent, approval, authorization, order, registration
      or qualification with any governmental authority is required for, and in
      the absence of which would adversely effect, the legal and valid execution
      and delivery or performance by the Seller or the Collection Agent of this
      Amendment or the performance by the Seller or the Collection Agent of the
      Sale Agreement, as amended by this Amendment, or any other Transaction
      Document to which they are a party.

      Section 8.2. This Amendment may be executed in any number of counterparts
      and by the different parties on separate counterparts and each such
      counterpart shall be deemed to be an original, but all such counterparts
      shall together constitute but one and the same Amendment.

      Section 8.3. Except as specifically provided above, the Sale Agreement and
      the other Transaction Documents shall remain in full force and effect and
      are hereby ratified and confirmed in all respects. The execution,
      delivery, and effectiveness of this Amendment shall not operate as a
      waiver of any right, power, or remedy of any Agent or any Purchaser under
      the Sale Agreement or any of the other Transaction Documents, nor
      constitute a waiver or modification of any provision of any of the other
      Transaction Documents. All defined terms used herein and not defined
      herein shall have the same meaning herein as in the Sale Agreement. The
      Seller agrees to pay on demand all costs and expenses (including
      reasonable fees and expenses of counsel) of or incurred by the Agent and
      each Purchaser Agent in connection with the negotiation, preparation,
      execution and delivery of this Amendment.

      Section 8.4. This Amendment and the rights and obligations of the parties
      hereunder shall be construed in accordance with and be governed by the law
      of the State of Illinois.
<PAGE>
IN WITNESS WHEREOF, the parties have caused this Amendment to be executed and
delivered by their duly authorized officers as of the date first above written.

                                    ABN AMRO BANK N.V., as the Agent, a
                                    Liquidity Provider and as the Enhancer



                                    By:    /s/Kevin G. Pilz
                                           -------------------------------
                                    Title: Vice President



                                    By:    /s/Thomas J. Educate
                                           -------------------------------
                                    Title: SVP


                                    AMSTERDAM FUNDING CORPORATION



                                    By:    /s/Andrew L. Stidd
                                           -------------------------------
                                    Title: President

                                    SWIFT RECEIVABLES CORPORATION



                                    By:    /s/William F. Riley III
                                           -------------------------------
                                    Title: Vice President


                                    SWIFT TRANSPORTATION CORPORATION



                                    By:    /s/Gary R. Enzor
                                           -------------------------------
                                    Title: CFO
<PAGE>
                                   SCHEDULE II


           LIQUIDITY PROVIDERS AND COMMITMENTS OF COMMITTED PURCHASERS


NAME OF LIQUIDITY PROVIDER                             COMMITMENT
--------------------------                             ----------
   ABN AMRO Bank N.V.                                 $183,600,000

ENHANCER
--------
   ABN AMRO Bank N.V.                                 $ 20,400,000
<PAGE>
                     GUARANTOR'S ACKNOWLEDGMENT AND CONSENT

The undersigned, Swift Transportation Co., Inc., has heretofore executed and
delivered the Limited Guaranty dated as of December 30, 1999 (the "Guaranty")
and hereby consents to the Fourth Amendment to the Sale Agreement as set forth
above and confirms that the Guaranty and all of the undersigned's obligations
thereunder remain in full force and effect. The undersigned further agrees that
the consent of the undersigned to any further amendments to the Sale Agreement
shall not be required as a result of this consent having been obtained, except
to the extent, if any, required by the Guaranty referred to above.


                                    SWIFT TRANSPORTATION CO., INC.


                                    By:    /s/ Gary R. Enzor
                                           -------------------------------
                                    Title: CFO