[kylie]

Just to play safe, take it easy
Its not fun when a shoulder pops out (Believe me, I have helped my friend pop it back in and its so weird)
This is normally for improving posture but here is an idea that I think will help your shoulders but have never tried it.
Walk 3 miles per hour on a threadmill or around the block and cross your arms in front of you and imagine your pulling your shirt off, so raise your shoulders to the sky. Keep doing this until you feel confortable with your shoulders. I think this move will push your shoulder blades towards your spine so it may help you.
But as I said before, I have never tried it, it came to the top of my head when I read your post.