Transmission / Driveline problem

I have recently replaced the auto transmission with a used one in my 1997 M3 (auto). I just finished putting everything back together and filled it up with fresh fluid and have run into another issue.

While the car is still sitting on jackstands, I can easily cycle through the various gears and everything sounds / seems fine. Wheels turn as they should AS LONG AS I HOLD THE BRAKE slightly. However if I release the brake completely I’m getting a significant driveline shutter. If I feather the brake while in any gear including reverse ¼, ½, ¾, pressure, wheels turn as they should. Shutter begins as soon as I remove the brake completely. I cannot tell if this is coming from an unbalanced driveshaft (center support installed slightly off?) or does this sound like a problem with either the torque converter or transmission?

Thanks

It sounds like the drive shaft is causing this because it is out of balance. Whether it is caused by some drive line issues or possible installation issue is tough to say.
I don't know if the guibo being bolted to the drive shaft is critical or not but looks like it would be worth checking too...