Jack the Corvette up and place on 4 heavy duty jackstands.
Or use a car lift.
Check the rear suspension, U-joints again, & U-joint straps & bolts.
Spin the rear wheels by hand with the transmission in Neutral.
Have a helper if need be.
Try and isolate the clunk.
Check the Gear oil level and condition of the oil. See if its low and gear oil is rancid broken down.
Many times the gear oil in the diff needs changing with fresh new. And fresh limited slip Posi Additive added to.
2 bottles of the current GM Limited Slip additive is whats recommended now on top of the gear oil.
Add the Posi additive 1st then fill with gear oil.
75-90 w.
Or 75-140 w.
Some guys only like Mineral grade gear oil. GTR1999 is one of those guys.
I use only Royal Purple Synthetic Gear Oil.
Or Castrol Type C Mineral grade gear oil.
75-140 w is my pick today on the street and what I use myself.
Drag Race only 85-140w.