Start with the easy stuff first. I'd get the tires balanced on a Hunter GSP 9700 tire balancer. Follow this link to read more about their force balancing and locate one near you: http://www.gsp9700.com/
Their balancing is lightyears beyond the old way and will give you smooth-as-silk cruising!! It's really sweet! If that doesn't take care of the vibration, then I'd look into the subframe bushings and rear end, etc. Good luck!
Edit: another thought: do you still have the convertible shakers in your car? That can cause vibration problems if they're missing.
Ben's collection of Super Sports:
- '67 Camaro RS/SS350, dark blue, black deluxe int, 4 spd - nickname: NemeSS
- '96 Impala SS, black, all stock, wife's daily driver - nickname: MASSTR (of puppets) (pics soon)
- '91 Chevy SScort, silver, auto, 4-dr hatchback, looks strikingly similar to a Ford Escort
[This message has been edited by No 'E' in Camaro (edited 06-27-2001).]