bad vibration

So by high rpm in second do you mean moving at the same speed of 65 to 70?
That may be a stupid question. But we are trying to rule out tire problems.
If the engine and transmission parts are changing speeds but the drive shaft, rear and tires are not that is what I am talking about.
Look at what doesn’t change.
Does coast or power make a difference?
I had a Challenger I gave up on.
Got rid of it anyway.
Had an annoying oscillation at high speed.
Drive shaft checked and double-checked.
489 all new bearings.
I think it was harmonics or tires.