Just my experience:

My TMJD, clenching, and grinding seemed to be directly related to neck tension. After muscle relaxers stopped having any effect Valium was the only thing that helped. The Dentist wanted $500 for a custom guard but amazingly it all went away temporarily when my surgeon gave me a course of Toradol. After rib resection surgeries, I haven't had any further TMJD or grinding. I do still have to be careful not to clench when I am concentrating heavily on a task.