ive got a 2003 f150 4x4 extended cab on 35s with 3.55 and im wanting to swap to 4.56 im guessting mine is 8.8 with 31 spline? how do i know for sure. is the front and rear the same? and when purchasing parts will 1 master install kit work for both front and back or do i need to buy 2?

 

4.6's have a 8.8 rear, 5.4's have a 9.75 rear unless you have the 7700 package (7 lug wheels), those have a 10.25. The front is a 8.8 reverse cut.

 

glc is right about the rear sizes. Quickest way to check yourself is to count the bolts on your differential covers. 9.75" rear will have 12 bolts on the diff cover, 8.8" will have 10. You should have an 8.8" Reverse-cut front. You'll need 2 master install kits, 1 for each differential. 4x4's generally need matching front and rear setups.