1981 Corvette, stock, 4spd..

Recently replaced all u-joints, re-built trans, replaced clutch, replace both rear assemblies (brake, shocks, entire rear assembly, left and right, due to bearing seizure on the left).. Also, re-built rear end.. Underside of the vette is all new, front to back..  Started noticing a sound when releasing the clutch ( a clunk).. Sound is getting louder and more frequent.. Seems to be coming from the rear..  Any ideas where to start looking?  We put the car up on a lift and did a visual, along with shifting gears.. No sound unless car is under load stress..

When the diff was rebuilt, did it include new stub axles?

Dave