I agree with PapaJ, My 1990 Chevy was taken to the local Chevy dealer and a "new" rebuilt trans installed. When I took it home, around closing time, it vibrated at a certain speed. Don't remember what speed but it was low. I took it back the next morning and was told there was nothing wrong. Well two dealers later a shop manager, who happens to own a beautiful 1955 Chevy car, rode with me and told me to go on and bring the speed up. Around 80 to 90 mph. Bottom line. drive shaft was 1/2 turn out. Fixed the issue and no trouble since.
