Any ideas on how to free a rusted ball on B&W hitch?

If the ball is directly above the 3rd member jack up the body of the truck not the rear end until the tires come off the ground. Wedge a 4x4 or 6x6 between the rearend and the ball. Let the jack down and go for a drive around the neighborhood. Hit a few bumps you could even have someone taping on the top of the ball while driving.