Wolfe - sometimes with engine/trans mounts, when you have a vibration like you describe... it can be the the engine is "cock" slightly. IF after a few hundred miles you still have a vibration - I would loosen the top bolts to the engine mounts and low trans/cradle mounts. Just loosen the nuts a bit. Then start and stop the engine a few times to rock the motor. Then tighten everything back up and go for a road test. This often times fixes mystery vibrations like you described above.
PS: Do you have any photos of the new mounts?