The manual states that the rear differentials don't require the fluid to be changed, but since I had my truck in the shop earlier this year for a CEL, I went ahead and had them change the rear differential, transfer case, transmission, and radiater fluids changed at ~100k miles. I believe the transfer case requires fluid change at 60k miles for extreme use. Normal use the front diff. transfer case, and transmission require fluid change at 150k miles.
It's very rare that these diffs leak fluid but you do at least want to inspect them once in a while because a leak could cause damage.
I currently have 125k miles on my truck and I probably will have the serpentine belt changed this year and will probably have the front diff fluid changed at the same time.
My 05 Super Duty I never changed the front or rear diff. fluid in the 180k miles I owned that truck and both diffs. were bone dry when I sold it.
