Back Squat Mobility

[FlatsFarmer]

Here’s a success story.

Shoulder Pain in Straight-Bar Back Squat Position

Not a bulky person but agony when reaching back to this position, mostly felt at the base of the delt between bicep and tricep on the right shoulder. Done some of the tests but can’t decide what it is. Side lateral raises ain’t so good, but other movements not affected, benching is no problem, Diagnosis? Rehab? TIA