91 corvette problem way out of hand, HELP

No longer own the car...

The mileage is 105,000, the latest update is this, we put in a brand new flywheel and a new rebuilt converter and now the 1200 rpm shake is gone, only to be replaced by a violent 2000 and up rpm shake! I thought it might be a bad converter so we got another, put it in and it was still that same nasty shake. I have checked with transmission experts everywhere and all agree there is nothing inside the transmission that could make that much of a shake. We even checked the crank for end play and it was tight. Also checked for an out of round pilot hole at the end of the crank, still nothing wrong. Another transmission guy suggested to make sure the dowels that line up the transmission were properly positioned and they are. I am at a complete loss of what to do now. ANY IDEAS AT ALL WOULD BE WELCOME AT THIS POINT.
Prior post:
The engine in my 91 corvette has a very noticible shake around 1200-1300 rpm. No shake at all above or below this range and car runs excellent otherwise. This happens while the car is standing stiil in neutral or park. Replaced the harmonic balancer ( vibration damper) with a brand new one and nothing has changed. Disconnected the torque converter from the flywheel and it stops. Has anybody else seen this problem?


1. sounds to me like a bad u-joint in your driveshaft?
2. or a bad motor mount