Gents - If you zoom in on Robert's pic with the missing shackle pieces (I snapped a pic and attached below), that spring mount is still on the frame and there isn't any evidence of welding on either side of the truck rear end. Rivets all seem to be in place and frame mounts secure.
Robert, I would venture to say either the cross plate bolt broke or the outer nut did loosen and come off leading to loss of the shackle side plates. I would 2nd Joe H's guidance on the shackle installation process. Change the other three to the proper installation method and replace the missing one the same way. The addition of the inside nut probably led to the outer nut not having sufficient tension to stay in place, so leaving that out is a must.
Glad to hear there wasn't catastrophic damage or injury involved in the loss!
