Hey Enigma:
I'm so sorry to hear that you've been having to deal with that level of discomfort. It's difficult to get a clear picture of what treatment plan would be best for you without actually taking a look at you. I can definitely recommend a few stretches that I think might help, but I'll give the disclaimer that everything should be done within your comfort level -- stop if there is any pain that doesn't feel like "good pain."
I think a great way to open up that underarm area is to perform a "door jamb" stretch, which is a favorite of many physical therapists and massage therapists. Begin by holding your arm out at a ninety degree angle, with your forearm pointed towards the ceiling. Find a door jamb and place your forearm against it. Step forward into a lunge position. You should feel the stretch in your chest and shoulder. Hold the stretch for ten to fifteen seconds, and then switch sides.
YogaJournal.com has a really great resource for specific stretches with an anatomical focus. Specifically for the underarm area, I would try the following asanas:
Extended Side Angle Pose
Cow Face Pose
Aside from stretching, I truly believe that ice is a miracle. Physiologically, it accomplishes a lot of what a good old fashioned Swedish massage can -- it squishes out old blood and allows for new, healthy blood to come in. As cold as it is outside, I would highly recommend icing the area to decrease pain and increase the vitality of your tissue.
Lastly, I would encourage you to take a look at what activities you are engaging in that might be causing that area to tense up more than necessary. See if you can go through your day with some added awareness of how much tension you're holding on to in that shoulder. Can you soften it as you sit at the computer? Can you drive your car with just a little bit less effort? Stretching is a great way to move towards health, but I would urge you to take it even a step further and see how much ease you can bring into that area of your body.
Take care of yourself, Enigma! Thanks so much for writing in.
Warmly,
Sara