Sorry, I have been incommunicado for several weeks trying to get some writing done and end-of-year stuff. @TrainForPain Thanks for the tag!
Everyone here has made many significant contributions, and a reasonably comprehensive book could be written just from the materials everyone mentioned. The videos and recommendations, including the book are excellent (John Rusin-like approach).
The shoulder is the most complicated joint to treat in the whole human body. The fact that the joint is so mobile also means it is inherently the most unstable joint in the body. Thus it is most susceptible to various forces and potential injury. Even if you do not have overt pain, oftentimes, there are deficits in ROMs and weaknesses that are not discovered until an injury occurs.
Because the shoulder joint is so complex, neuromuscular control is very intricate. The shoulder has one joint attaching it to the main skeleton (the sternoclavicular joint), and each muscle crossing the “ball and socket” shoulder (glenohumeral joint) is associated with at least two other joints and some more than a dozen (i.e. the traps and lats). Most of the shoulder mass is soft tissue, delicately balanced to provide range of motion, stability, and strength.
I say all this because the shoulder relies on the rest of the body to have adequate mobility, stability, and strength to operate correctly and be healthy. More so than any other joint in the body, in my estimation.
Here is my point… DO NOT get caught up in looking only to the site of pain (ever) for a solution. In the time between the current pain and getting a diagnosis or working to fix it yourself, find and improve other areas of the body. Concerning the shoulder, concentrate on the pelvis, spine (whole spine), breathing, and elbow/hand. By fixing mobility issues, posture, and breathing, you often will find improvement in shoulder function without more focus on the shoulder.
I will post a series of evaluative motions and muscle tests here soon, so check back.
Continue to work towards solutions for the shoulder. Get a diagnosis with a good diagnostician. Here is a link to find one near you: Find a Provider - Active Release Techniques
Look for this certification: