[Lynn Eades]

Mike, without the pin locking in the hanger, the pin movement will occur between the hangers and the pins. This will happen rapidly because the surface area is relatively small. Not only that, but the fix is difficult because your hangers are riveted to the chassis.
The whole idea is for the movement and wear to occur between the shackle pin and the spring bush which is set up for greasing at that point. Also the bushes are an easy replacement along with the pins.

Note that the nuts for the shacle pins are castleated so as to be located by a split pin. This leaves some free movement at the ends of the bushes relative to the hangers. This helps to prevent spring breakage.
Maybe you could try an interfearance taper and step at the pin head? Just a thought.......