Ok.. I have a 2001 GT and it wasn't a concern really but I'm going to PA soon and will be doing more then 70mph at times I'm sure.. Anyway I figured it was one of my rear tires so I just went and got 2 used but good condition and replaced them balance etc just for the trip.. I do 70 and bang it's still there now what is curious about it is after 70 the more I hit the gas the more it vibrates BUT if I let off on the gas cruising at say 80 it doesn't vibrate bad but if I just feather the throttle it starts again which makes me think it's got something mechanical to blame.. Any ideas?