I had a vibration problem after removing my trans for a flex plate issue a couple years back. I ran this motor/trans for about 6yrs and installed it as one piece. After reinstalling the trans, separate from the motor, I noticed a vibration only at certain RPMS. I did the following....this procedure is in a factory troubleshooting guide someone on Moparts sent me when I was trouble shooting.....

After breaking in your engine........

1. Loosen motor mount bolts
2. Loosen Trans mount bolts
3. Fire off motor
4. Power brake motor a few times....more if required
5. Take measurements.

If the thing is straight and level, retorque all mount bolts, back car into street and put the heel to the steel!

-71STROKERFISH