My wife and I purchased a 71 challenger 440 six pack 4 speed car, that was originally a 318 automatic car. We both enjoy working on the car but we are stumped on this one.

The car drove well when we originally purchased it but had a noticeable vibration at 75mph for the first year. We later noticed the car was bottomed out on the lower control arm bumpers. We ended up replacing the torsion bars with 0.96" bars and we were able to get the car ride height corrected (the old torsion bars were actually swapped right and left and were clearly worn out). We took the car to a chain store to get the alignment done afterwards and ever since then we have had a violent vibration that starts at 45mph that seems to settle out at 55 mph and have nearly no vibration at 60+. The vibration occurs at these speeds even if I take it out of gear. Also, I believe it is in the rear as I do not feel it in the steering wheel.

I thought the problem was the tires as they were 10+ years old, so we ended up changing both the wheels and tires, but still have the same issue. I was wondering what the next troubleshooting step would be. Any help would be greatly appreciated!