This is classic and what you are describing is called the "Death Wobble" in the Jeep community. I cannot tell you what is wrong but I can tell you what causes it and where to look as there are a few different possibilities and even if you described it to me in more detail would not help.
Check the following:
Control arm bushings, worn tie rods, worn ball joints, worn wheel bearings, and any other steering components or bushings.
Jeeps are known for their Death Wobble and it is due to worn or broken parts in the steering linkeage because of their design. Do not listen to people who tell you that it is just your steering dampener as it could be bad but is not the cause of your problems and is only a bandaid fix.
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Thursday, March 4th, 2010 AT 8:00 AM