To help debug the issue I would try using a GoPro camera and watch the suspension and linkages from different angles doing various test runs
The video may show something non intuitive happening.
I had a bad wobble once on a mid 70s pickup
It was a combination of a cheap tire and bad shock absorber.
Replacing either one stopped the wobble but I replaced both in the end
I was curious if it was just one of the two causing the issue but it required both the junk tire and bad shock to work together to get the wobble going
It could be pretty bad between 35-50mph if you sped up the wobble would go away
-s
