Other than the ART, what kind of treatment have you had?
The area of your problem contains many different structures that could be pain generators.
In addition to the rhomboids, you have the mid-traps, thoracic paraspinals and several different intersegmental muscle groups. Also, you have the thoracic spine facet joints, costovertebral and costotranverse joints. While thoracic spine disc problems are not as common as lumbar spine disc problems, this would be something else to try to rule out.
Any of these structures could potentially produce pain in that area.
You should look for a good chiropractor in your area. One that deals with athletes and can address the joint problems as well as the myofascial problems.
Often pain between the shoulder blades can be related to having rounded shoulder posture especially if your daily activities require a lot of flexion dominant positions.
After you get checked out, I would suggest to look at Mike Robertson's and Eric Cressey's Neanderthal No More article series.