I had exactly the same problem, because the coupling broke off after the pin was removed from the front shifting tube. It seems the threaded end had some threadlocker on and it simply snapped where it was drilled and weakest when I unscrewed it.
A thin nut, like Roger suggests, now locks the coupling to the front shifting tube, whereas I left the rear pin/tube untouched. It was easy to adjust and hasn't loosened in 10 years.