I would suggest chocking the front wheels, then jack-up and place stands underneath the rear axle and support car.
Then, start engine, place trans in high gear, and check straightness(or lack of) of the rotating driveline.
I found a bent U-joint this way after checking by conventional methods.
Driveshaft should rotate smooth, might even remove wheels and see if vibration is gone, Substitute a driveshaft, wheels, brake drums(?), etc. Could be an out-of-balance clutch and/or clutch cover or missing clutch material on plate. Does it happen with engine RPM in any gear or with speed of car only?
I don't think it could be a bent/warped frame, but then I'm not a body/frame man, either.