Let's nail down some points so we don't have to keep going over the same ground. 1. There is no single cause, it's always a combination. 2. Ideal scrub radius is about 2". 3. Scrub radius has a direct effect on the tires thru the road force applied by rolling along. That force is counter acted by the toe adjustment to keep the wheels straight. Increased scrub radius increases that force and a necessary increase in toe to compensate. 4. Bump steer will cause/force the wheels to turn as the suspension moves. 5. New parts doesn't mean good ones. Sad fact. 6. I've personally been delivered spindles with bushings reamed that were way too sloppy. 7. Caster changes Mix up some of that , like on the video. Rolling along just fine and Scrub is applying even force to both front wheels and the toe is keeping everything at bay. Hit a bump and the addition of factors within the above combination starts. the bump steer cause the wheels to turn, toe goes all to one side, the road force is suddenly diffent side to side, and then the tires try to redistribute the forces back to equal side to side. The tires aren't trying to balance out a small amount of forces, they are trying to balance out something within that combination of extrordinarily huge forces. Not only that the forces are dynamic which equates to over compensation in an attempt to re balance forces at the tire and now you have the death wobble. Throw in a few more in your case. The tube axle with stiff radius rods, the parallel rear ladder bars are causing some stress and loads onto the chassis. Those stresses are induced and then unloaded, stress is energy and creating stress is creating energy. That released energy is going to go someplace and it wants out. Where's it going exactly??? Small parts of it have heat as a safe escape but the majority of that energy doesn't have a safe outlet like heat. With that anti roll bar action the frame rails are getting loaded and then unloaded like a steel rubber band.