I've been working for over a year, off and on, to regain the shoulder flexibility to do the back squat. I've had great people, like Mike Robertson, help me out with ideas.
Finally, finally, tonight I was able to assume a mostly comfortable high bar position.
Scap mobility. Internal rotator stretching, external rotator strengthening. Pec and lat stretching. Rhomboid and middle trap strengthening.
It came down to all of that, but the squat specific squat I devised with pvc and bands, behind the neck presses (very light) and actually squeezing under the bar on a daily basis are really what made this happen.
Now comes the rebuilding of the movement. 135# felt awkward and strange, and still a bit painful. It's going to start to come back more quickly now with daily practice.