I just went through this after having mine in for a few years. To make matters worse, I had drilled and welded a 5/8" nut on the side, rearward of the pin, to allow for a bolt to be tightened down on the male portion of the receiver to eliminate "roll" of a dirtbike carrier and slop when hauling a trailer. I twisted off the bolt so had to grind the nut off.
After a few weeks soaking with PB, I still could not get it to come free, even when tied off to a telephone pole.
I finally got mad and heated the hitch to the point that the paint was smoking. Tied the truck off to the pole, put it in 4L and applied the max pressure I could without spinning on the gravel drive. Put it in park and started wailing on the hitch, side to side, with a sledge hammer. It finally started inching out.... Well, millimetering out.
I wire brushed the whole unit, applied liberal paint, and will no longer leave the hitch in place.