Mine was changed at about 5k with the install of a new ring and pinion
I changed again recently at 35k with Royal Purple. (I should have done it again after R&P breakin but didn't)
The fluid was quite funky and had more solids than I would have liked to see.
After 3k miles on the Royal Purple differential fluid alone I have noticed allot of tire slippage under load in turns.
I am going to be adding the Ford Friction modifier.
If it sounds too good to be true it probably is.
Royal Purple Gear Oil
"All viscosities of Max-Gear are formulated with hypoid friction modifiers necessary for use in clutch or cone type differentials. No additional additives are necessary."