Intermittent rumble/shaking of 2007 Grand Cherokee
 

This happened a couple of times. I start the car, leave my driveway and the car is acting as if I'm driving over a highway warning cut into the roadway (those lines they cut into the roadway to warn you you are drifting). The car is vibrating. Then it gets worse and feels almost as if I'm driving on a flat tire. The car is shaking badly. Then it goes away and the thing is fine. My mechanic told me he cannot do anything until he drives it with the problem. I guess he doesn't know where to start. The car has almost 150K and I have loved it since the day I got it used in 2012. I'm new to this. I hope I posted properly.