What is the latest year model f150 defferentual that will fit on my 2003 f150 xl 4.2 with a 5 speed manual transmission. 2wd

 

A 2004 Heritage body truck (not the boxy new body ‘04 trucks) with another 8.8” rear axle (behind either a 4.2 or a 4.6 v8) in it.

 

Any 8.8 rear that has all the same hardware and Brake hookup to match yours between 97 and 03.
As you mention Differencial; in these rears, only the gears are replacable. The center is all one unit with the axle housings.
Otherwise you would need the whole rear axle unit.
If your intending to replace just the ring and pinion, better to review first and have the Dial indicator , shims, and specs for the job.
Good luck.