Joe - Try squirting it down with PB-Blaster., Then have a helper hold pressure on collar in the off direction , while you lightly tap collar forward with a flat punch / hammer . They often get rust around the balls and collar when they are not exorcized frequently ; or lack of lube. This always works for me , then I clean / lube real good when I get it un hooked .
Or if you don't have a helper ; you can hold the pressure on it while using a pry bar on the collar . Usually once you get it over its' initial sticking point , it will go the rest of the way pretty handley .
Hope that helps . Dave