Is the diff oil full? could be the axle bearings getting hot.
I am still learning the GM platform and could be way off, but...
I don't think diff oil is supposed to be making it past inner axle seals to lube axle bearings.
I would expect rear diff fluid to stay in the pumpkin where it belongs.
A frequent scenario that does happen a lot is:
If a caliper locks up and over heats the rear rotors (or drums if so equipped), the red hot rotors transmit the heat to the axle shaft which can sometimes get hot enough to melt the outer rear axle seal and might actually cook the grease out of the rear axle bearings. Then bearings burn out and and seals need replacing.
Sometimes you get lucky... Like I did!
Bearings and seals managed to survive glowing red rotors when my calipers locked up with new pads and rotors I just put in.
The caliper pistons had been so far out for so long, there must have been corrosion building in the cylinders behind them.
When I pushed the pistons in to make room for the fat new pads, the pistons would move out but not back in to release the pads.
Managed to save pads and rotors, believe it or not! Replaced the calipers and was back in business.