A shake that appears and goes away with speed is usually caused by an out-of-balance wheel. This happens with my Jeep (at the same speeds you're having issues with) and when it does I have to rotate the tires and get the front tires onto the drive axle.
It sounds counter-intuitive, but that's the first thing I would try. If simply rotating them off the drive axles don't help, have their balance checked.
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Last edited by dyn085; 02-24-2014 at 02:26 AM.