Another exercise
I echo the comments r.e. strenthening the read delts and rhomboids. Also, the inferior (middle & lower) traps retract the shoulder girdle, so work can be done there.
A good exercise to isolate the rhomboids was suggested. I would also suggest doing "shrugs" in a horizontal plane, i.e., by pulling you shoulders straight back (arms pointing straight ahead at 90? with your torso). You can do these on a seated row (Hammer makes some good ones) or a T-bar row with chest support. (They can be done w/o chest support, but its more difficult to have a focused controlled movement.) Keeping you arms straight (slight bend at elbows - not locked out), simply pull your shoulder blades back and together. In your case, I would not try to have an extreme stretch in the movement (you are already unbalanced in that direction), but try to do the repetitions with a very short controlled mvmt. near the fully retracted (fully contracted) position.
Good luck. Oh, yeah - lay of the front delt work, too. 8^)
-Randy