Search "samurai death wobble" in google, it will bring you back to a great wealth of information on this forum and a few others. Have a peek and see if this sounds like what you are experiencing. I hope this helps.
o boy, u call this the (death wobble) several things can cuz this namely... bad steering box,small universal joint after wheel,bad shock damper, bad tie rod ends . it happen to mine and the back suspension rubbers were the culprit.i only found out after i put some very heavy load at the back of my sammy dats when i realise it didnt shake much and it arouse my suspicion and i change the rubbers (those which have to be machine pressed) and wa lah it was over.
Your experiencing death wobble. First thing to do would be check your tires. I have seen numerous jeeps get this with out of round tires or improperly balanced tires. From there check all tie rods and bushings. If it looks in any way odd replace it.