I think you're describing a situation in which the pin threads covered by the nut have succumbed to corrosion. Essentially there are no threads on which the nut can advance to the exposed threads.

If the nut spins freely on the pin then there's a chance the ball joint will dislodge in the course of driving. The nut should keep it from coming off completely and the pin is probably nicely cold welded to the spinde, but over time it some play can build and cause noise over bumps.

BTDT - I just fixed that problem in a sway bar link in the Caravan that spent its first 100K miles in VA.

You should be able to borrow (AutoZone) or rent (anywhere else) a nut splitter. Or if you're really crafty, pry between the nut and the spindle as you turn the nut. You might get it to catch a good section of thread.

Sixto
95 S420
87 300SDL