[Bob D.]

In Jeep circles this is known as "Death Wobble." I've experienced it myself and it does get your attention. No one I know has pinned down the exact cause, but I believe it's a combination of things--is your jeep lifted?
Are the suspension parts worn? Does one caliper seize on occasion?
Sorry, there is no simple answer. The front suspension needs to be checked over by a competent mechanic.

Here are some possible causes-

Track Bar worn or loose
Bad Hubs
Tie Rod ends worn or loose (either drag link or Tie Rod)
Alignment issues
Tire Balance Problems or out of round tires.
Control Arm Bushings.
Steering stabilizer bad.
Worn Steering Box