REar diff seal is leakin really slowly. Goin to replace seal and fill it back up. How much do I need and what kind it's also limited slip. Thank you

 

Take a look here:

http://bfy.tw/4f1

 

Oh me Tackle, that was classic!!

 

75w-140 synthetic fluid, you will need 3 qts (well, 2.6 or so, but get 3 qts). Also use the Ford Friction Modifier ( ), even if the fluid you use has FM in it. These LS clutches are picky and many have had to end up adding it after the fact (which is harder to add after it is filled).
Now as for which brand gear oil, well, I am cheap. I was going to use the Super Tech stuff from Wal Mart (~$12/qt), but found some Lucas for cheaper on Amazon. IMO you really get no benefit from Mobil 1, Royal Purple, Amsoil etc over cheaper gear oil for what these truck are used for.

 

My 2002 Owners manual calls for: 2.6L of SAE 75W-140 Full Synthetic.
I like Lucas or Amsoil 75W140 and a bottle of Friction Modifier too (for LS).

 

Higher end fluids dont break down as quickly and actually run cooler.