I have had the same issue with mine. When inspecting underneath nothing appeared to be loose but everything did have some wear to it. After checking tire balance and alignment I started by changing tie rods ends and even the pitman arm because it was worn. It narrowed down the wobble to a smaller section 50-65mph. After this I replaced ball joints and while I was in there I pulled the axles and did the ujoints and wheel bearing assemblies. This again tightened everything up and really made the tj ride nice but I still got a narrow band death wobble at about 50-55 constant speed if hitting a bump. This next week I will be replacing the trac bar and if that doesn’t fix it I will replace control arms. I know I am in to this for a bit of time and money but I can say that all of these components were worn enough that each has helped my TJ drive better and be more enjoyable. My Jeep has 165000 on it with much of it having been towed behind an RV. I plan to let my kids drive this when they get to high school so it has been fun having them help do the work and learn about the components. In the meantime I get to make it safer, drive it, wheel it, and put the good scratches in it.