What is your current flexibility/mobility routine for your shoulder?
I want all of it if possible to see if you're missing something.
Also, shoot me a call if you want.
Initially it seems like you probably want some serratus work (serratus punches for example), internal rotation stretches, and then some more specific external rotation stretches. Also, may need some rotator cuff work ebcause of other scapular dysfunction.
Depending on what you are doing currently you may need to modify a bunch of it. Post up what you're doing and we'll see....