I'm 41 months out from the end of treatment, and know precisely what you are experiencing. You don't have a signature, so I can't tell if you had a neck dissection or, if so, what type.

The radiation fibrosis gives that "wooden neck" feeling, but the fibrotic tissue can also impinge on nerves that run through the side of the neck, and produce a variety of symptoms, ranging from stiffness, to muscle pain, to spasm, to symptoms mimicking carpal tunnel. Physical therapy and stretching has been helpful for some. There is also some literature indicating that botox injections can be helpful, but I don't know anybody personally who has had these.

If you had a neck dissection, the problem can be compounded, due to trauma to the nerves, particularly if the dissection extended into Level V, where the Spinal Accessory Nerve frequently becomes involved. That can result in a spectrum of symptoms all its own, including trapezius and shoulder issues.

You're not alone!