It sounds like what you experienced would be a vibration in the drive train/braking system since it occurred when slowing. DW is encountered when you are driving along and hit a bump or pavement irregularity and it does not stop until you have almost brought the vehicle to a stop. While DW vibrates the entire vehicle you would feel it most through the steering wheel but feels like it is tearing the vehicle apart (it actually can).
It is caused by loose or worn components, most commonly the track bar which is attached to a bracket on the frame on the drivers side and the fixed part of the steering knuckle on the passenger side. The duty of the track bar is to keep the front axle from drifting left and right with coil springs in the front. The U-bolts clamping the front axle to the leaf springs did this on those vehicles.
If it is new, simply replacing the bolts on the track bar can fix it, but by the time it rears it's head enough it can need new tie rod ends on the tie rod and new ends/new drag link (connects the steering pitman arm to the movable part of the passenger side steering knuckle) as well as a new bracket and or new track bar. Some install a steering damper when the vibration first occurs, but that is treating the symptom not the cause.
But, the fact that the vibration is felt when you were slowing and the lights indicated another problem. If you were braking at the time, it may be warped brake rotors. With the lights on, it may be the ESC and or TCS modules. With 85,000 miles, it may be late in the life of the second set of brake pads. New rotors and pads are a relatively inexpensive fix if they are the cause.
If the vehicle is worth it to, another test drive, to a knowledgeable Jeep mechanic might be in order, if nothing else to read the error codes. If the lights are on, it should be throwing codes. I am more troubled by the fact that the dealer shows little interest in diagnosing, much less fixing the problem.
I would assume that you did not look under the vehicle for rust (but I bet the dealer did), and I would also assume that the dealer is not offering you a CarFax report on the vehicle.
Never, ever buy a used vehicle without a CarFax report (even if you have to pay for it yourself). It will give you the registration history of the vehicle and a sketchy maintenance history if done at dealers, as well as any accidents. Also never, ever buy a used vehicle without looking underneath for rust, especially the invasive rot of frame and under body. Minor surface rust can be treated and controlled, but if it has penetrated significantly, it can be expensive to fix.
I would also recommend continuing to look for a better vehicle, but adhere to the above paragraph. If a vehicle is priced at a bargain, then maybe it's not.
Here is a link to a thread on this forum explaining DW in depth. Something for you to review at your leisure. I have never experienced it on either of my Jeeps, the Rubicon is well maintained and was when I bought it with 112K miles on it two years ago and the Sahara is too new for it.