I suffered (still suffer) from a similar problem. You will need to do a lot of assistance work. Your external rotators are probably very weak as well.
You are not double jointed and do not try and work around the problem. Confront it by strengthening what is weak. Nothing sucks more than people laughing at you doing 5 pound external rotations but in a month you will double if not triple that and continue to progress.
Read about shoulder anatomy and take a hard look at what is not working. Then find some exercises to bring your lagging areas up to snuff. Surprisingly enough when I started doing this work all my upper body lifts went up. I can even do pull ups now without a problem.