Those are not called shackles and unfortunately tightening them up will not solve your problem. It may raise your bed up until you drive it one time and they will be spread apart again. Their purpose is to keep the springs aligned and should not even be tight. They do not hold the leaves together. You need to have your leaves re-arched or replaced. This is common for all of these old trucks, if they were used much at all they are now sagging.
