I agree with Big Chief's advice. I have a similar issue with my 57 Tbird at 45mph. I've narrowed it down to a bent axel or flange and since that's more than I am interested in messing with, I'm living with it for now. The way I narrowed it down was to swap the wheels around first to see if that changed anything. Next I had the driveshaft balanced and installed new U-Joints. When none of this worked I figured since I don't drive the car much I'll deal with it. If you want to drive it often I would jack it up and remove the rear wheels. Then have a friend start it up and put it in drive and get it up to where it should be vibrating. Watch the drums and see if everything runs out smoothly. Also it is possible a balancing weight came off the drum. A good machine shop should be able to check balance on the drums.
Hope this helps.