Volvo AWD XC Forum

"What makes this assembly not rebuildable?"

For one, no one sells any internal parts for it. I've opened them up before and there's really not much to it. It consists of a ring and pinion plus bearings and upon visual inspection, one won't SEE anything wrong by looking at it nor will its bearing seem bad either. The exact cause of what causes them to become noisy is presently unknown to me.
I forget the total amount of fluid but the ONLY way to know if it's even low is to first drain all its fluid and measure that. One needs to drill a hole through a lower bolt hole to even drain its fluid so I'd question its "being low" unless it had a leaking seal. They will normally expel some fluid from its upper vent hole if it gets too hot and the fluid expands enough to spill out.
As for used parts with "30K miles", amazingly ALL used parts from salvage yards somehow all have "30K miles" on them. I'd highly doubt that unless you see the car and its odometer itself.

Sounds like you know about the tire related issues so then that's then not the cause with you but did you EVER use the space saver spare tire? Believe it or not, using that for even a few blocks can cause bevel gear problems.