[mokoosh]

The common arch/sag/bend of the tailgate seems to be the result of weight placed on it while it is down. That makes me think it could use some structural support internally, going across from side to side. If it is cut open to repair the bend, that seems a perfect time to add some bracing to prevent it from happening again.

I agree. The drawback would be added weight and the risk of warping the panels with additional weld points, as far as I can see.

I was toying with the idea of a piece of aluminium pipe glued or simply crimped into the bottom cavity.