take a look at the motor mounts themselves.
1. on both sides, make sure the cast mounts are not resting or touching the frame side of the mount anywhere.
2. how tight did you attach/torque the bolts? is it possible you over-tightened the bolts?
3. is the motor evenly centered between both mounts, in all directions?
4. if you put the trans in neutral, and rev the motor up, does the motor move in the engine compartment?