Hello Mr. Scott,
Thank You for your query at DoctorSpring.com
Do you have a habit of sleeping prone? Or with your arm under your head? That is one of the commonest reasons of what you're experiencing right now. The muscles go into chronic spasm, there is a lot of pain around the affected shoulder joint and it takes a long time to resolve.
From my own experience, i had the same habit, until i woke up one day with the same kind of muscle spasm you described, severe pain around my shoulder joint, and some persistent numbness mostly around my elbow and forearm. And it did take a long time to recover, nearly a month.
You have already tried most of the basic measures like anti-inflammatory and muscle relaxants. If they don't work, chiropractic treatment and acupuncture are the only alternatives. If they don't work too, you need to sleep supine and straight for the time being, and it will gradually resolve. The best way to resolve this spasm is acupuncture, you can go for hydrotherapy ( where epsom salt is added to warm water ) and you can sit in the tub with the mixture, it slowly releases the tension, and is considered one of the better ways of relieving spasm in the neck area. The acupuncture specialist will be able to arrange that for you, and if he has not mentioned, you can suggest hydrotherapy for yourself to him.
I feel hydrotherapy is the best way to go about it, since all other methods have failed.
From my own experience, and having tried similar techniques like yours, it takes close to a month for the pain to subside.
Hope this was helpful,