[tonythetarga]

What year car?
You might try removing the rear sway bar completely and you might discover the mount is cracked or broken. You said you did check that but unless you had both back wheels off the ground at the same time, you may have missed it. Also check the drop link bushings for the sway bar. somethimes they can break or the rubber bushing can be worn out and allow the bar to clunk. No problem to drive the car without the rear bar, as long as you aren't planning to track it in that condition. I'd start there; it is an easy remove-replace part to diagnose the problem.