[mr48chev]

Leegreen hit the nail right square on the head. The movement is in the shackles.

The factory style shackle pins on the 56 should have tapered ends that seat in the arms so you can't do the welded up version but you could take an accurate measurement with a caliper and cut some heavy wall tubing to length and insert it in the middle over the bolt to stiffen it up. If you didn't replace the shackle bushings and pins when you did the redo they may be worn to the point they are loose and those pins and the rest of the chassis have to be greased every 1000 miles or more often. There is no such thing as greasing an AD or TF truck too often.