You can also try to put a little sand in the socket. I've used this many times working on trucks. Just enough to get a good grip. That is usually my first choice, then the pipe wrench or vise grips. As mentioned above if that does not work then weld a nut on. Just make sure you have a new one to replace this one with now. Also, once it's out, examine the threads because it may have been slightly cross threaded as it was put in.