I've never seen these bushings you are referring to that were missing, Any I've ever seen were always metal on metal.

To me it looks like you may want to replace the piece of metal the pin is welded to, on the box side as well. I would fab new pins, and weld them to a new piece of 18 gauge that is in the shape of that flat part the pin is welded to, from about an inch above the pin to the bottom of that flat. Then cut out the old piece and weld the new piece onto the box side and the inside brace, grind it smooth, then paint it to match the box. I'm not sure that can be done without removing the rear bumper.

This is a pretty standard fix on old pickup trucks, of all brands. Not including the bumper removal (if it needs to be done) and the painting, the whole process takes about an hour per side.

If the flat piece the pin is welded to is too weak, it will allow the pin to drupe down under the tailgate weight, causing the pin to wear prematurely. Gene