I have a vibration or more of a rumbling feeling in my car whether it is in gear or not at about 2500 rpm. Would a good guess be the torque or tranny area? The engine seems fine.I had a similiar rumble in my 1977 Lemans after installing a clutch fan.It is not constant but more in intervals.

Remove the torque convertor to flexplate bolts and then retest for the vibration. If its not there then you have a tranny issue, if it still is then its an issue with the engine.