If you dont mind getting a new front end alignment, you can remove the
through Bolt that holds the upper control arm to the frame.Use vise grips
to hold the end steady , while you remove the big nut. At that point you
can bash away at it without obstruction. Souldnt need to go that far
though.Maybe a screwdriver as a wedge will work I thought this was
sunny,corrosion free, Southern California.

Im not sure, but maybe you can put the wheel back on and  gently lower the
Vanagon to the ground.Let that pressure push the ball joint out.