can't search numbers, anyhoo, is the 9.75" rear the same as the pre 04?
 

and are the 4.6 and 5.4 rear ends the same?

04+ F-150's with 5.4L have 9.75" and the 4.6L trucks have 8.8"

Yes the 9.75" rearend is the same 9.75" rear end that as offered in pre - 04 models..

It most definately is NOT the same axle. The spring perches and shock mounting points are completely different.

Internally, the gearing, carrier, and pinion may be the same, and the axle shafts may also be the same. However, the axle housing itself is not.

-Joe

is the 8.8 a 28 spline or 31 spline?

Check this website to help answer your questions. There is some good info/calculators etc!

http://www.ring-pinion.com/DiffList....F150&Side=Rear

I think the 8.8 is 28 spline

I know the Rangers use the 28spl & the 2nd gen Explorers used the 31spl. So I'd say the F-150s use the 31spl also..

On the website I posted above it states:

Rear applications have c/clip axles in both 28 and 31 spline. Front applications in late model IFS Ford's and will have 28 spline axles using CV joints.

how do i know for sure what spline count i have?

Call a dealer and give them info on your truck. They should know for sure what your truck has in it.