My Jeep is shaking violently when I get up to about 40_45mph. It's a 6 spd. I have to put brakes on put it in 2nd or 3rd gear to get it to stop. When I first bought it 2 yrs ago it would shake but nothing close to how bad it shakes now. It is very annoying and try driving it on the highway what a joke! Sometimes I can get through the shaking and get it up to 65. Anybody know what the probkem is?