Heat is my first thought. That always seems to work well to free a metal to metal sticking problem. I would heat the socket area while trying to keep the heat away from the ball. Hopefully the socket will expand enough that tapping on the ball will free it. Just make sure there's nothing flammable under the pickup bed near to the hitch plate.
Or take a large pipe wrench with a piece of pipe on the end for leverage and try to wiggle the steel block back and forth to loosen whatever is holding it in place.
Or if you have access to a gooseneck trailer I would connect it to the ball, drive around on a (not too) bumpy surface to help loosen and then use the trailer landing gear to raise up on the ball
