I fought my vibration problem for months and with too much $$$ replacing parts after parts.
I also had fairly severe vibrations between 50 and 70MPH.
New tires, tripple balanced, switch wheels, new flex disks, new center bearing, tripple check suspension components, balance drive shaft, etc, etc. . None of it changed anything.
I found a new ''old', specialist who diagnosed weak engine supports. He was so sure of his diagnostic that he told me he would not charge me anything if this would not fix the problem.
A few hours later, the engine mounts were replaced. VIBRATION TOTALLY GONE"
It's been over 70K since then and the car still runs very smooth.
That may be your solution.