This reminds me of one of my trucks several years ago.
I replaced the gear oil with synthetic at around 140k miles and thought nothing more of it. That was until I hit 190k miles and lost the rear end on the HW. The shop opened it up and said I had no fluid left inside. I never saw a leak and they did not note any signs either. Even if it were a slow drip or seeping, I think I might have noticed.
I've always wondered where did the fluid go and how long did it run w/o any? Then I really started second guessing, like - did I even remember adding the fluid after the drain?!