Buckling bronco!

If it's "bucking" the hitch itself is actually sliding forward and backwards, which means the hitch pin is smaller than the hole, allowing for movement of the hitch every time you apply brakes, or accelerate from a stop. Over time, the hole that goes through your receiver hitch and the hitch shank will wear bigger and bigger, allowing for more movement, which will make the "bucking" even more pronounced. It's just the nature of the beast.

"Bucking" has nothing to do with porpoising. Bucking is forward backward motion. Porpoising is upward and downward motion.Porpoising occurs because of weak springs, no weight distribution bars, or just extremely bumpy roads.