I'm still trying to find ways to help you guys - How about some rigid extensions at the top of the ramp where it overlaps the tailgate. This would distribute the weight away from the weak tailgate lip further towards the front of the truck or closer to the pivot axis of the tailgate?
Cub - I think 2 grown men can sit on the tailgate just fine, since their weight is on their a$$cheeks towards the middle of the gate as opposed to the edge. I think the only problem would be - why would they want to ?
