My friend has a 03 F-150 5.4 auto 4x4 with 3.55 with non limited slip. the offroad shop near our house has a 07 f-150 5.4 auto 4x4 limited slip rearend unit. now the question is the 07 axle was 9.75 it had 3.73 gears the custemer replaced that with a truetrac and 4.56s. now my friends rear axle is 9.75 are they inter-changeable? will the unit just bolt into the 03? the offroad shop said it would or should. it is just the "pig" does it matter that it had 3.73 and going to be bolted to 3.55s? and will axle shafts going to be the right lengh for the clips? All constuctive input is appreciated. Thanks
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PIG question????

Where does the term PIG come from, and how does it relate to your rear end?

Some 9" axles had the center section moved a bit one way or the other depending on the application, and it might or might not be the same with the one you are looking at.

Measure the length of the shaft and OD of the splined end.

Then count the number of splines.

If if measures up, yes, it will fit.