Jeff,
You are bang on my friend. Shoulder pain and/or numbness in the shoulder, arm and even hands are a more common occurence after Neck Dissection than radiation alone but, patients who recieve only radiation can also have similar experiences.
Radiation to the neck can cause fibrosis of the nerve sheaths and surronding tissues leading to scar tissue and nerve impingement. The nerve bundle that is affected and gives rise to these symptoms is the Brachial Plexus . I have attached a link to an anatomical illustration of the anatomy of the Brachial Plexus. If you look at the position and extent of this nerve bundle you can see how it could easily become affected by treatment/surgery in the neck region.
I hope this is useful to some of you.
Cheers,
Mike
http://intraspec.ca/images/brachialplexus.jpg