The dominant shoulder being lower is something that is very common, especially in one-side overhead athletes such as a baseball player and in swimmers as I believe someone stated earlier. This is something that I suffered with as I played baseball at a competitive level from the age of 5 until a few years after college. I have added some extra external rotation exercises (Shoulder) a few times a week as a rehab/prehab type of thing as well as adding a few sets of dips where I focus on completing a reverse shrugging motion at the top of the dip while "puffing" my chest out. Also, I will do just the shrug and back up motion, as I would at the top of the dip, by it self. This has helped considerably with this issue, as well as the overall health and stability of my shoulders.