I have a 99 F-150 with a 4.6L and was wondering what size rear I had. I was hoping 9" but have a feeling it is and 8.8. is there a site that has the codes that are on the door panel sticker and what they mean? Any help appreciated.

 

8.8" with 31 spline axles.

the 4.2L & 4.6L get the 8.8". The 5.4L gets the 9.75".

 

Ford hasn't used the 9" axle under trucks since about the mid 80s.

You can also look at your axle tag to make sure as well. Mine with 3.55 diff and 8.8 axle reads something like this
3 8.8 55 O or at least something along those lines.

You can also tell the difference by looking at the diff housings. The 8.8 is more of a sqaure shape with very rounded corners on it and the 9.75 is almost more of a hexagonal shape. If you look at them side by side its pretty obvious.