It's a VERY common problem, and could have many different causes, and solutions..
For you reading enjoyment.. I searched the Grand Cherokee forum for "shimmy", and got a kazillon hits..
They also call it "death wobble"
This is a very busy, underpowered website, so you may have to wait until off peak hours to use their search function..
One of the most common problems is with front end alignment.. Set the toe to 0*.. You can do that in the driveway, with a tape measure, piece of string, etc.. That would be my first place to start..