I'm by far no expert and have been looking for advice regarding my back problems that started out with a stiff neck.
You mention you are a golfer and golfers are very susceptable to pain/problems in the thoracic area. If you are certain it's muscular pain, something my physio recommended (that eases my pain and stiffness) is a heat pack/ wheat bag or whatever you want to call it. It's a wheat filled sack often lavender scented that you stick in the microwave for 2-3 mins and place on the affected area. It's no miracle cure but does ease things a little.
I also find standing most comfortable but not for too long, sitting is the worst. I can't really offer you much more advice than that. I have never suffered with a bad back before but now I really can empathise with anyone who suffers with there back it's not pleasant and I really feel for you.