I've been waiting for this rainstorm to let up so I could take a look. Checked the ABS, wasn't the culprit. Had a thought pop into my head about the AWD system, and how weird it behaves when an axle pops out. Left rear was about 1/4" out of the diff. Popped it back in and it's back to normal.
Looks like it's popping back out as soon as I pull it out of gear. Must not be seating for some reason.