I have a 64 M38A1 (1990 4.3L Chevy, stock T90/Dana 18 and axles) I just installed a 4 inch superlift. I used new shackels from performance accessories that were 1.5 in longer than stock. Since the M38A1's came with shackels reversed, that only helps the caster. Anyway, the problem is that when I get upto 30 - 40 mph, then hit a bump on one tire or the other, the wheels start "wabbling" back and forth (left and right) and I'm not positive, but it also seems like they are hopping (right tire up, left down -  then left up, right down) at the same time is the wabbling. When I say wabbling, I mean some pretty violent shaking is going on and it gets rapidly, proggressively worse until I slam on the brakes and almost stop. My first thought was bump steer, so I designed and installed a 3 inch drop bracket for the bell crank assembly, which restored all the tie rods and linkage to almost exactly stock orientation. Front end alignment was just done after the drop bracket was installed. What the bleep is going on? Please Help.  

sounds exactly like the problem i'm having with a stock 96 f350. I did all the balls and the problem got worse. dont know if it's a caster prob. or if it's the steering box. I'm hoping that a stabilizer kit will help tell the tale.Those shackles u used may have screwed ur cxaster instead of helping.when u figure it out, let me know