I also suffer from upper back pain on the same side as TMJ. This has been present for long time even before TMJ. It is in the subscapular (shoulder blade) area up to the neck. I also notice that my neck muscles and even soft tissues/glands are inflamed very often.
Also, the shoulder often pops in and out too, especially if I rotate it around, unlike the other side, it is less stable, and is prone to pain if I lay on it.