I have a 95 LS teg. I had the a similar problem a while back. I replaced the motor mounts put the polyurethane mounts in, and replaced the CV shafts and that fixed it. It was vibrating really bad. Now I noticed it just vibrates some as in the whole car shakes under load. The more load you put it under the more it does it. Its not even as bad as it used to be but Im not sure what it could be now. The CV shafts are no more than 4 months old and same with the mounts. I havent replaced the rear tranny mount yet though.
Any opinons on why it would do this?