2004 Jeep Wrangler Jeep shakes at high speeds

In my experience with this problem also known as death wobble there can be several culprits.

Badly worn or leaking stabilizer shock.

Tires out of balance, if you are not sooo particular about looks and can get it done have the weights distibuted on the front and rear of the wheel as this is the better form of balancing.

Mud or dirt inside the rear of the wheel. Caked on mud will cause the tires and wheels to be out of balance a quick trip to the local carwash may solve this problem.

Ball joints
Tire Rod ends
loose lug nuts
Sway bar issues
the list is endless the first 3 are the most common and probable the easiest to fix. Also how are your tires wearing are they cupped or scalloped air pressure is the culprit here.