That sounds like a rear tyre out of round to me - possibly a flat spot or a slight bulge in the tread surface. You can easily overlook that on a balancer as the wheel is covered by the guard, and it can easily be balanced out.
I would jack each rear corner up and rotate the wheel by hand, feeling for any deviation in the shape. With one hand turning the wheel and the other palm on the tread surface you can generally feel if the surface changes. Or, swap the rear wheels to the front and see if it moves to the steering.
Another possibility is a bit of rust or grit between either the wheel hub and the disc, or the disc and the bearing hub, which is causing run-out on the wheel once its mounted on the car. This is generally too subtle to see without a dial gauge so if moving the wheels end to end didn't help I would look at pulling the rear discs and making sure all the mating surfaces are spotless.
The propshaft spins too fast to be described as a wobble. If thats out of balance it tends to show as a vibration around 40-60. By 70-80 its too fast to feel in my experience. The prop will tend to straighten itself due to rotational inertia so a duff centre bearing is usually felt as a thump through the floor on over-run.
From your description, definitely something to do with the rear wheels or how they attach to the car
