One of the tires/wheels on the rear is the problem, that’s why the vibration moved from the steering wheel to the seat of your pants. Move one tire back to front on one side of the car at a time and that should isolate the one tire that is the culprit.
The tire could be defective. The balance of the tire could be so far off the shop should dismount the tire and try again. There should be a heavy mark on the wheel and a heavy mark on the tire. A good installer will find these marks and put the heavy mark on the tire opposite to the heavy mark on the wheel.
Finally the wheel could be bent, or if the wheel is an aftermarket wheel it might be missing the “centering ring”.
Sometimes it can take a while to identify and correct the problem with a “vibration”.