pain between neck and shoulder--what exercises can strengthen and help relieve stress in that area?

You need to see a doctor about this. It might not be stress. It could be a nerve issue, or something else. Rule out any possible medical problems before you start doing exercises - you could cause yourself serious damage.

try massage. take some almond oil, make sure you are not allergic to nuts though. I use about 30mls almond oil 4 drops of lavender oil, and also add 5 drops of Rosemary oil. put it into a dark glass bottle. not plastic, make sure you are not allergic and that you like the smell. massage it in small amounts, you will get relief, the massage really helps. try to keep movement in the arm too. If you don't move it may seize up and be more painful. Try to use the oil at night before bed, and you should feel relief in the morning. Use good essential oils. Or you could just use some slippery oil, not vegatable, with the lavender and Rosemary, and really work into that knot. I get it when I have been too long on the computor.

You should definitely get checked out by a doctor first. A few quick and easy things to do would be to stretch your chest at least 5 times a day. You can just stand up anytime at work and take 2-3 minutes and stretch it out. A really good exercise would be the standing or seated row. These two things in combination will really help un-round the shoulders and take pressure off your neck.