If bringing the revs up to the range where it vibrates while sitting still in neutral, and you sense the vibration, it could be in the motor as well as the flexplate or torque converter on an automatic, and perhaps as far back as the input shaft of the tranny. All this stuff is still rotating while in neutral.
I am chasing something similar in my T with a SBC 350/TH350. I can go down the road in the RPM range where it vibrates, slip the gear in neutral and it instantly goes away - put it back in gear and as soon as it hits the 'right' range, you can sense the vibration again. In my case, the range is 2000-2500 rpm. Also, sitting still in neutral, you can feel it in the same rom range. It is not a strong vibration but if you are looking for it you can feel it.
I originally thought balancer, but the balancer on it now looks good with no visible cracks in the elastometer (still could be the balancer though I put that at a low percentage). I've eliminated pinion angle and driveline issues. I've pretty much determined that it is either a flexplate / torque converter issue or I've hit a natural harmonic with the frame. This will be looked at more closely this winter.
Good luck to you.