Shooter - I'm sorry to hear about your problems. I've never had vertebra fused, so I cannot comment on that. I am recovering from a rotator cuff tendon problem, and wen through extensive therapy (including weights) to strengthen the right muscles in my shoulder.
That being said, I am 61, and a day on my Surly includes the usual dose of stiff necks and shoulders. Sometimes, at the end of the ride, I almost feel like the Hunchback of Notre Dame. I have worked with my LBS, and they wanted to get me more upright, and a little closer to the handlebars -- to limit the shoulder reaching and the neck bending. Second, I have regularly included one or two weight sessions a week, concentrating on core strength, shoulders and neck, as well as steps for legs (which has nothing to do with this). Third, I religiously stretch after a ride - if my neck or shoulders feel stiff, I will stretch at a stop (doesn't take more than about 5-10 minutes so I don't lose much time).
Long winded way of saying - you might want to talk to your LBS about this, and maybe a sports medicine MD.