The rear differential should be changed early if you have the LSD, in my opinion. Lots of break in/wear stuff, and my change showed it too, as I mentioned earlier. The E-Lockers and open diffs can probably go a bit longer, but even they will have some wear metals in them.only 17k miles and you changing all those fluid already?!
The others I change and fill just for my peace of mind, plus the fact that the factory underfills them all a bit to save a few bucks. And they're easy to get to and refill on our trucks. The transmission, on the other hand, I may top off (referencing the underfill I mentioned earlier), but otherwise I leave that alone until at least 50k miles.
100% necessary? No. But I rest easier knowing my expensive contraption has better fluids.



