Just went through the same type of vibration drove me crazy 70 challenger 727 8 3/4 276 highway gears shake would start at 70 and stay hard to about 90 would seem to smooth out after that but still there. Tried different shafts Trans mounts pinion 2 degrees down ,tires & wheels ,put it on jack stands got to 75 @3200 rpm shook bad removed the tires same removed drums no shake DONT STEP ON BRAKE just shut off the car and put in neutral put tires on with out drums tested once more on jacks with just tires & no drums no vibe. Got 2 used drums put on it's like a new car no vibe. The drum that I took off looked fairly new & had no weights. Good luck