[Sirrotica]

I would bet it's the driveshaft, but for grins, look in the center of the harmonic balancer to see if it some soil got packed into it.

Many years ago I had a 65 4 speed GTO, that had the same symptoms, and the vibration was in it when I bought it. I finally swapped the driveshaft out of another 65 Tempest, smooth as silk. I looked that old driveshaft over plenty of times, ran it with the rear wheels off the ground, and never could find anything wrong with it, but swapping it, changed like night, and day.