Slightly Embarrassing Question

You know, if you drive her up to 65/70, put the trans in neutral, and shut off the engine, then if the vibration is gone, it almost had to have been in the engine or trans.But if the vibration remains, it has to be from the mainshaft to the pavement.
So the next thing to do is ride the brakes, in order to drag the pinion down and worsen the the angle. If the vibration gets worse, then the pinion angle or U-joints MAY be the root of it. Or perhaps you just introduced a new vibration that was not there before. But if braking reduces the vibration, then the pinion angle would be suspect.
I have used the car-on--jackstands-with-wheels-on-and-off, several times. You can watch for; wheel hop (Tires out-of-round),wheel imbalance,rim and tire runout,oval drums,a bent axle,and pinion jacking.