By replacing the transmission mount, you changed the angle that the driveshaft runs on. You might read my post about vibration that I put up last night.

Since you're vibration is occurring at slow speed, you are dealing with a different Gremlin than did I, but it should be good food for thought.

In a conventional, solid axle car, a misaligned driveshaft shows up as slow speed vibration.

Also, I'm pretty convinced that the vibration beginning after changing the transmission mount is no coincidence.

Hope these thoughts and this information is of use.

Have a great day,