This is funny because I think the service interval is 100k miles on the diff...
If you drive the thing like a grammy, never push it, barely engage the electronic locker, never take it offroad, & never hammer it onroad WOT on the reg than 100k intervals should be fine for ya
Personally...I like to swap out stock fluids @ advanced intervals so I can drive w/ peace of mind knowing it has the right stuff in it & running it hard isn't causing excess wear. I also prefer to do an initial fluid swap earlier than most b/c I have personally found on a number of occasions the factory used the incorrect fluid volume or even TYPE (in the case of K2 GM truck trannies & TC fluid) - so its not a bad thing to do. Now I can do it in a snap myself too - having to entirely remove a diff cover & have that nasty shit dump all over my garage follpwed up by a chnitzy ford gasket & RTV? No thanks. Such a Ford thing though - nonsense to not have a drain/fill hole in truck diff.
FWIW - this cover won't rust (not that my stock would anyhow I painted it & sprayed down w/ HDMP) since its got a heavy duty powdercoat, supposedly cools the internals down faster, & tangibly holds more fluid = good when you're pushing the truck hard.
I change the engine oil every 5-6k, front diff & transfer case @ 22k along w/ the rear 1k miles later - I'll continue swapping the fluid in rear diff every 35k, ill swap the front diff & TC again when I have the tranny fluid swapped @ 50k & then lengthen out to every 50k for those components & stick to 5-6k engine oil & 35k on rear diff
Similarly I installed a fumoto drain valve w/ the swivel so I can drain oil w/o dropping covers or making a mess - Ford exacerbates this ****** design even further by requiring the oil filter be screwed in from the side vs. the bottom. Must have alot of Germans working for em these days I guess.