I agree with most of what I read.
6"with 3" spring, 3" spacer, and no drop bracket is old school.
OK for the trail, but not optimal for the street. Harsh ride and poor geometry are the highlights of that kind of lift. Also, the 3" spacer, is like a poor man's bump stop; you will limit your up-travel to what the spring can compress to, which is also hard on the spring.
If you want to stay at 6", you may want to either move the mounts back, like in a long arm setup, or use drop brackets to get the geometry under control. Then consider revising your spring set up to full length coils. The last XJ I had at 6" had long arms, and no issues other than it handled like a circus wagon.
As DW can be caused by anything from wheel balance to worn components it can be problematic to find/locate. My 1st thought after seeing that the problem showed after trip to sand country was sand in the wheels. You didn't mention your steering setup. The stock XJ steering rods are great for the XJs original mission, but with larger tires act like a spring. If you haven't upgrades, consider a ZJ setup, they are fully swapable, and is a solid arm. Tire sizes are also a huge factor. Up to a 33X12.5 shouldn't cause much of an issue, but above that the sidewall is a bit flexible and I've seen a lot of folks in that size with DS they can't fix.
