I will tell you right now it is the upper rear knuckle bushings. The one(s) between the upper control arm and the knuckle. When mine first started I was like " Oh shit, WTF?" then I checked it out and said screw it. Then the other side went, I then bought ES bushings, Installed them, and guess what? No more freaking clunk over bumps, RR tracks, under hard braking with warped rear rotors, etc., etc.

BTW, Holy Old Post Batman!