so i'll be riding along on the highway and when i decide to pass vehicles or become a target for police and speed my car makes a horrible vibration only between 75-82 mph's ... after i get above the 83 range my trailblazer floats like a Cadillac. the problem sucks because the speed limit here is 70 so i cant go 7 over its either way over or obey the law and i've had to obey the law and that isnt any fun. any ideas on what could cause that just at that speed? its consistent it does it everytime and never fails to vibrate at those speeds