I have not read all of the posts here so I don’t know if this has been considered:
My Wagoneer had a serious wobble condition that got worse as the temperature increased. In fact I could not drive it above 35 MPH if it was 90 deg. F outside. The problem started about five years after a complete vehicle restoration. These two clues led me to suspect rubber. I noticed one of my spring eye bushings had deteriorated so I made two new ones from polyurethane and installed them. This cured the problem.