The frame-side bushings are a pain to change, and their outer sleeve is press-fit with the leaf spring eye. That said, they are removable with persistence... and would be a cheap rule-out. Might as well pull the whole spring pack and check all the leaves for cracks. Replace the U-bolts on reassembly -- IMHO, these are best used only once.

IMHO, your locker is amplifying whatever *squirm* you have -- and unless you are actually connecting the rear tires together to equalize air pressure, I wonder if they are exactly the same, and that's assuming the tires have identical wear. Have you tried rotating tires to see if there's a different.

I don't understand why changing the shackle changed the handling... dunno. Try repeating it to see if you can recreate the perceived change.

Randii