It could very well be the motor and trans. mounts. If it is sitting lower than required the axle geometry will be off, thus causing a vibration. If you take a look at your motor mount (under the balancer area) and see a bunch of black liquid goo then your mount has failed, as they a fluid filled from the factory.

FYI- There is no "back" motor mount on these cars. There is a motor mount up front under the balancer, a trans mount on the drivers side (under the trans, duh, lol..) and the 2 dogbone mounts you see when you open the hood.