Since you have the ability to put it in the air. I would check your wheel bearings and rotor runout to see if they are loose or out of true.
Wheel bearings in the rear are shimmed to get the clearances very tight. 0.002" - 0.004" total play in the bearings. It's not a job for amatures, as it requires special installation and dis-assembly tools to get the proper bearing clearances.
Remember that the brake rotors are mounted to the bearing spindle, so any error in installation of the rotors will be amplified as the wheel is mounted flat to the rotor hub.
Generally speaking, most mechanics are not familiar enough with rotor replacement on C3 Corvettes to do the job right. It is usually better to leave the rotors alone than to replace them. Too late now.
First step would be to get the wheels off the ground, and try to wiggle the tire in and out at the sides, and top to bottom. If it moves side to side and top to bottom, the wheel bearings are probably worn out.
If it moves only top to bottom, then the traling arm bushings and/or the top traling arm linkages are worn.
Since you have the ability to turn the wheels and observe the wheels and rotors in action, you should be able to see if there is an excessive amount of runout or wobble in the assembly. Then once stopped, check for looseness in the wheel bearings and traling arms.
One other possibilty is that the bearing spindles themselves could be worn or bent, which would cause a substantial wobble in the wheel.