What is causing my car to shake when driving on the highway?

What is your question?

My 2009 Jeep Grand Cherokee shakes violently when driving above 60 mph on the highway after I let the car sit for a day or so. When the shaking starts I bring the car to a complete stop and the shaking subsides. I have brought the car to multiple dealerships and mechanics and no one can diagnose or fix the problem. Please help!!!

What makes this problem better or worse?

bringing car to a stop makes the shaking stop

How long has your 2009 Jeep Grand Cherokee had this problem?

months