A more serious response to Fbody69's idea:
The vibration is an oscillation like a sine wave. The amplitude gets greater as the speed goes above 65 or so. The wavelength however is longer than the rotation of the wheels, driveshaft, u-joints, bearings, engine, etc. It has to be a resonant frequency of some sort. So I really couldn't guess what it is that way.

One interesting clue that I don't know what to make of, is that the oscillation changes when I put the car in neutral. The wavelength gets much shorter. It must be changing because the driveline is unloaded, but it's still there because of the vehicle speed and not engine RPM.

I'm on the west coast right now so I can't do anything until this weekend.

Okay, professional vibration analysis guy. What do you think of them apples?