[Max777]

so it's not rpm specific, but it IS speed specific? Ok, that helps a little.

Have you put the car up on jacks or a lift and inspected the underside for any obvious damage?

The driveshaft has stick on weights, if one of those came off, it would come out of balance, or if it got dented.

The loud clunk makes me think that a diff mount might be a problem here, however dont rely on this alone, as there might be another problem. You could try to do the pinion snubber mod, and see if that helps at all. It does wonders for wheel hop issues.