As you have already found out, Death Wobble, the horrible front end vibration that starts when one wheel (usually the right) hits a bump around 40~50mph, is the worst possible downside to having a coil-sprung vehicle with a track bar setup. Vehicles affected by this design are the Jeep Cherokee XJ, the Grand Cherokee ZJ and WJ, Jeep Wrangler YJ, TJ, and JK, and also include Ford and Dodge trucks and early Ford Broncos. Death Wobble is also extremely difficult to try to diagnose, because it is actually caused by slop in the entire steering system as a whole, not by one component. To diagnose correctly, your mechanic needs to look for "play" everywhere there is something that could have "play" in it. It's time consuming, and downright dangerous while you are in "test phase", trying to exorcise this demon from your Jeep or Truck. 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 (also called a Panhard Bar Internationally, as well as a tracbar, trackbar, and trak-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 a front end 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. DO NOT let the alignment shop talk you into a four-wheel alignment, as this is only useful on vehicles with independent rear suspension in my experience, and since there are no adjustment points in the rear of a live-axle vehicle ANYWAY, you're just paying for a service that you won't get anything out of. If you have a lifted vehicle, make sure that the alignment shop you choose knows the variant specifications for lifted vehicles, and that they do NOT set it to the "default/stock" settings. A good quality alignment shop familiar with 4x4 vehicles will know these settings, and a poor quality shop will likely tell you that it doesn't matter whether it's lifted or not...they still use the same specs. Hang up the phone and call the next shop, if so. If you are *certain* that the front end alignment 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 factoryfront track bar. Over time, the tie rod end on the upper portion of the Panhard or track bar (some applications like the WJ Grand Cherokee, the Ford and the Dodge have a rubber bushing instead) develops "play" in it, and the same goes for the lower end, which has a rubber or polyurethane isolator bushing in it, which allows the Dreaded Death Wobble oscillation to occur. Aftermarket trackbars 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 rig in general...NOT good. The reason why I created myTrack 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. A complementary product we have found to work VERY well in getting rid of Death Wobble is our exclusive Hard-KOR brand SuperDurometer Track Bar Bushings, which are also available for theTrack Bar Conversion. They are about 2x as hard as the typical polyurethane bushings that come in most aftermarket track bars, which are typically made by Daystar or Energy Suspension for the various aftermarket manufacturers. The polyurethane bushings 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 forewarned. The next thing to check is your steering stabilizer. I recommend replacing the stabilizer at the same time as whatever worn components that you find under the front end, as this "combination punch" is very often more effective than the change caused by each of the parts alone, since Death Wobble shakes EVERYTHING, and loosens up OTHER components at the same time. I've found that replacing the stabilizer my itself often times doesn't eliminate death wobble directly, but that it usually helps with some additional poor handling characteristics that cause the onset of Death Wobble, such as wandering, and a new one seems to tighten up the entire steering system. I ONLY recommend the OME SD40 stabilizer or our Hard-KOR Steering Stabilizers, because it seems to be tighter and work better for stopping Death Wobble than the other stabilizer models on the market. They are also the most heavy duty that I'm aware of. This stabilizer is something that I recommend to everyone who is having trouble with Death Wobble, because it's one of the least expensive parts to replace. Here are some other components to check over for looseness or improper movement: Tie Rod Ends, ball joints, Track bar mounting bracket bolts, steering box bolts, and track bar ends. Another product that we've made due to a need, is our XJ Steering Box Brace for the Jeep Cherokee XJ, and our ZJ Steering Box Brace for the Jeep Grand Cherokee ZJ, which holds the steering gear box tightly on those two models, allowing the additional stresses of running larger-than-stock tires be directed to the frame rails, rather than to the three little bolts that hold the gear to the frame, which get loose and if they do, will snap and leave you stranded. Another source of Death Wobble is over-inflated tires (you should have around 30psi in stock tires and far less the larger your tires are. See Boyle's Law and consider how much more volume of air you have in your 33" tires compared to stock. I run around 18psi in my 37" tires). The last thing that I can mention that has caused Death Wobble in the past is hub bearings. If there is a little slop in them over the years and miles, they MAY indeed help to cause the oscillation as well. I mentioned them last because they are the most expensive to replace and least likely to be the root of the problem. While you have the front end apart, you should consider adding some offset upper ball joints to your Jeep if you're running a lift kit, in order to return the caster back to what a stock Jeep would be if you are running say 4" of lift or more. Be aware that on full-time 4x4 models, changing the ball joints to the offset type tends to add more vibration to the front drive shaft since you are also turning your pinion angle downward in conjunction with the caster angle improvement...you can't have one without the other, and on the full-time 4x4 models, you'll get a little vibration at highway speeds by doing so. Compared to Dreaded Death Wobble, however, this is a VERY small price to pay, lol. Hopefully this short checklist gets you started on the right foot and helps to cure your Death Wobble.
Very Sincerely, Kevin