My G20 has turned into a buzz box

Last year I had some ordinary tune-up work done on my '93 G20 (automatic tranny). When I got the car back, there was a really irritating buzzing sensation (a feeling, not a sound) coming through the floor pan, gas pedal, and steering column. This happens when my foot is on the gas and the car is moving. It is RPM related however, not rolling related -- i.e., if I take my foot off the gas it's fine and at idle it is fine. The car was once satin smooth, but on the highway it is now a Geo Metro-caliber buzz box. Several mechanics have looked and looked but cannot find the source of the problem. Motor mounts, exhaust, etc. are fine. Anyone else had this problem? Anyone have any ideas? Thanks.