The thing I tell people to start with is a visual inspection.  Spend 10 minutes under the front end and visually inspect each one of the steering components for shiny steel, which would be indicative of metal that's moving around when it's not supposed to be.  Pay careful attention to the track bar, as it's usually the culprit in most cases.  If any of your bolts are even the least bit loose, Death Wobble can manifest itself and make your life a living hell.

If everything looks to be "normal" and you've checked bolt tightness on the track bar, the next thing to do is to start with an alignment, making sure that caster is set correctly as well as toe-in.  If you have been offroading and have bent your tie rod even slightly, it'll throw off the alignment.  Plus, it's only $40 or so.

If you are certain that it's set correctly and you've not replaced any other front end components recently that may have caused the oscillation to begin, I tell people the next most suspect thing is the factory front track bar.  Over time, the tie rod end on the upper portion of the bar develops "play" in it, and the lower end has a rubber isolator bushing in it that allows the oscillation to occur.  Aftermarket bars generally come with urethane bushings that allow much less "play" in the way of movement than the factory rubber bushings do.  The problem with most aftermarket track bars for the Jeep is that they also come with either a heim joint, Johnny joint, or tie rod end on the upper end of the bar, which works fine for a while, but wears out over time, leaving you right back where you started, with a mess in your shorts, a temporarily deaf right ear from the wife screaming, and the frustration with your entire Jeep in general...NOT good.  The reason why I created my Track Bar Conversion for XJs and ZJs is to eliminate that problematic (and expensive to have to replace when it wears out) joint as well as for the additional flex benefits.  If you are running an XJ or ZJ with more than 3" of lift, I highly recommend you check it out.  Another product we have found to work VERY well in getting rid of Death Wobble is our exclusive SuperDurometer Track Bar Bushings, which are also available for the Track Bar Conversion.  They are about twice as hard as the typical polyurethane bushings that come in most aftermarket track bars, which are typically made by Daystar or Energy Suspension.  They are also one of the least expensive replacement parts in the steering components, so they make sense to try first.  Many aftermarket track bars as well as the stock track bar are completely ineffective in managing Death Wobble due to their "effective angle of operation" if you are above 3" of lift.  Bear in mind that another alignment is necessary after replacing any front end components if Death Wobble still remains.  Sometimes you get lucky and don't need one...but be wary.