Does anyone know what type of fluid goes in the diff on a 1998 F150 4x4 off-road package? It says it is a H9 rear.

 

Should be 75w140 synthetic gear oil. Check the rear diff cover, there should be two tags, one is the part#,ratio etc and the other says to use 75w140 gear oil only. H9 is 3.55 l/s I think(don't quote me). If it is a limited slip you will need friction modifier also(if you are changing it).