Turns out, it WAS the driveshaft rear U-joint.
Got it all apart, and it was seized in one direction. Bought a new (reman-ed) one, and got it in yesterday (+ an oil change today) and it rides smooth as glass and there’s no more jarring jolt when shifting from R to D
Feels so damn good (minus my wallet) to have this thing run the way it should. Now (knock on wood) no more repairs for at least a few months.
Tried filming the whole thing on a go-pro chest mount, so TBD on a video