OK, had a chance to get the rears off and I'm now thoroughly confused. Here is what I found:
When I looked at the damage on the car and snapped those initial pics, I assumed the body had sheared off at the threads and just broken spectacularly - but that doesn't seem to be the case. Besides the dings and scratches from getting it home with it busted, I can't find any major damage and can't see any explanation as to how it got like that??? If I thread the lower portion right back in, there's no real resistance and it eventually marries up and looks/feels good (besides the cosmetic damage).
There's the two side by side. They seem fine, but I sure as hell don't want to reinstall the bad one on the vehicle until I understand what happened to it.
So, maybe my grasp of physics is just on Easter vacation today - but how is it possible that the lower portion simply worked itself out of the threads when all bolts were still tightened securely? The two bolts in the top hat were secured, meaning that portion can't rotate out of the lower - and the lower bolt goes completely through that housing, ensuring that it can't rotate at all away from the upper? What am I missing here?
EDIT: Probably solved below; I see how it could work itself free if the (aptly named) locking nut is not tightened well enough.