Well...I feel pretty dang stupid. It was the brake shoes coming unseated, which pushed the little parking brake switch slightly out, making it hit the wheel stud heads as they rotated.
The terrible noise in the video? That's the noise an open differential makes when you rotate one axle.
Since you have a GM, of course the carrier is open. That noise sounds to me like the way an open diff sounds when rotating one axle as you were causing the other axle to rotate via the spider gears assuming the trans was in park and the driveshaft could not turn.
Do you have any up/down play at the ends of the axles?
There will be some level of wear in any rear end at 126K.
On the bright-ish side, I found out that I need to change my differential oil again, and that replacing the parking brake components is a slight PITA, but doable.
I should probably still do my wheel bearings though...130k miles, and I bet they're original.