vibration problem

You have my sympathy. But I'm no help. No hair either. I’ve been there with a hot 340 72 Challenger. Finally sold the car, but not because of the “mystery” vibration. (But only above a certain speed) Tried a lot of things. Never fixed it. Could have even been tires. I know there are some things inside the vodoo science transmission that can cause vibrations.
I hope someone with drive train expertise can give you a hint of what to check.
My questions would be things like does it only vibrate under a load? Does it help to go to coast? You know that Chrysler put “vibration dampeners” on the tail shaft of transmissions. Most people take them off and they are missing.
NOT saying they are necessary. I don’t have enough experience to say.
Just saying. They were there for SOME reason.