Maybe stupid, but check your rims and make sure they're not bent. My 94 Firebird had a minuscule bend, almost unnoticeable, on the inside of one of the back wheels. I wouldn't really notice the vibrations until around 80 or 85. When I replaced the rim, the problem went away. Sounds stupid, I know, but sometimes we're so worried about it being something big that we forget to check the easy and small fixes.

 

after i installed my LS1 DS the vibration got reduced but at 95-100 the shifter still shakes. i'm going to take into consideration what wrd1972 said, and also check my mounts (thought they semms to be in godd condition) and i will check my rims to but they semms to be fine

 

its a camaro what do you expect?!