High Speed vibration, can't find what's causing it.

I got a good buddy out of a pinch years ago at our Bracket finals at maple grove in Pa. He had a vibration and i suggested putting a hose clamp on the driveshaft just tight enough that he could forcefully turn it. Make a pass then tighten it down where it stopped. It worked great and he ran it like that all season then addressed it over the winter. Something very quick to try. We put it close to the rearend.