[M_Six]

This past Saturday I took my brother out for a spin in the Zed. There's a very smooth and very deserted road here where you can see for two miles in any direction. It runs through miles of corn fields, which are empty this time of year, of course.

Anyway, we got up to 115 before I let off and coasted back down to 80 or so. There was a nasty high frequency vibration which I felt in the steering and through the shift knob from 115 down to 90 or so. I don't get it at all at more sedate speeds. Not till I hit 100 or so and it seems much worse coasting down from speed than it does under power. Could this just be a tire balance issue? Or might it be something worse, like with the tranny or drive shaft?