Buffybike,
All the replies you've read are good.....I had enormous pain between my shoulder blades & also my neck too. Stressful rides & hunching the shoulders was a real issue for me. Like Bikechick, my bike was too big & my custom Luna w/ a WAY shorter top tube has helped eliminate 90% of it (as well as my hand pain & girly bits pain).
I try, when I am riding along & know I'm headed for a big hill or something stressful, to lower my shoulders--I lower the elbows to lower the shoulders, or think of lengthening the distance between earlobes & shoulders. I try to relax.
Another thing I hadn't read on this post is this: look down the road / up the hill by moving your eyeballs, not by raising your chin. Try as much as possible to keep the neck vertebrae a straight extension of the back vertebrae. This helps me ALOT.
good luck
Sooz