My 1994 Plymouth Voyager (named "Rusty Jones") has had an issue since I first bought it around 6 years ago. I noticed that on the test drive, there was a pretty strong vibration at highway speed. I believed that since there was an extra set of tires in the back and since the owner bought the tires on-line that a balance was needed. Well, nothing worked to eliminate the shaking. New tires, alignments, new front end, engine and trany mounts changed with replacement of both, new rear drums and bearings. Cannot find anyone who can fix problem...Any suggestions?