peripheral neuropathy is another thing to consider.

Do you have the neck and shoulder stiffness primarily upon waking, or does it persist or come on at a different time of the day?

I have had neck and shoulder pain and joint stiffness upon waking for years. I take a lot of vitamin C and that has kept that problem to a minimum. These days I am taking 4,000 mg of C spread throughout the day. I did find relief years ago with only 2,000 mg though.

You might also consider trying the B vitamins to rule out methylation issues, though for me, who definitely responds to the range of B vitamins, it has made no difference on my shoulder and neck stiffness issues. My neck and shoulder problems are minimal once I'm maybe a half hour into the day.