Like Hoefler said, light tongue weights are the most likely culprit. I run into the same thing pulling farm wagons. Lots of weight on the wagon, but nothing on the tongue, and some of those wagons can buck quite a bit, depending on the road surface. Keeping my speed down helps, but does not resolve the problem.
I plated mine right off the get go and the hitch I use for pulling wagons has a solid shank, not a tube. Plating it before the problem started and using a solid shank spread the shock out over a larger area of the pin, resulting in less wear. As for the longer pin needed, that was easy: I use a farm wagon draw pin to hold the hitch into my receiver.