Just my thoughts. Bad wheel at speed. Bad Yoke or driveshaft , bad tire , not balanced , I doubt it it is the mods you made to the suspension unless the mounts have been changed and changed pinion angle ? Wish I knew more. Good Luck , I hate when that happens ! Just went back to this and think I have the wrong wagon LOL but the corrections are the same none the less.
Think I found the issue The original springs have her kinda squatting in the rear.. that's why I got new ones.. but the new ones also had her squatting the same in the rear. I just got under to look and the pinion angle is off and at a "V" My guess as to why it didn't shake bad before was from the slop in the bad u-joint somehow canceled it out. So I'll need to be returning my new springs and find someone that carries the verified correct ones.. should be fun