The FIR2255 Ride Rite Air Helper Spring Kit is designed for use on the 2WD model Excursion with a lower ride height where the 2251 is designed for the 4WD model Excursion with a much higher ride height. The bracket's between the two kits is different to accommodate the difference in the ride heights. Both kits will work on Diesel models but check clearance between the air bag and the exhaust pipe if you are running aftermarket exhaust. The bags want at least 4" minimum clearance but i want 6 to be safe. it is a good idea to fabricate a heat shield even if you have the minimum clearance.