Does the frequency of the "shake" increase with the cars speed, or is it just random vibration that is independent of speed? For example, the "clunk-clunk" of a bad CV joint will usually occur once per wheel revolution and can be heard to increase as the car speeds up. Is there any way to reproduce this on a lift or jack-stands (safely)? For example, if you applied power while holding the brakes with the rear wheels off the ground would you be able to hunt it down? Just some thoughts.