To compensate for any lack of shoulder range of motion when squatting I suggest using the safety squat bar. This is a great way to build up your legs as long as you don’t become too reliant on it. By this I mean if you aren’t able to comfortably get under a straight squat bar then it’s time to change your routine.
One of the ways to regain this lost range of motion is to use a four-foot dowel rod. Put it on your shoulders as low as you are able and then carefully lean into an open doorway. Let your bodyweight carry you farther into the opening while the dowel is held in place on the doorjamb.
Hold each position for ten to twenty seconds or as long as you can tolerate the stretch. Do not force yourself into this position; instead let the shoulder structures themselves tire out thereby moving you further into the stretch.
Take a look at the photo and you will get a clear picture of the exercise. The bar is in the low position and as I lean in I am purposely pulling it lower on my shoulders for an added amount of stretch and range of motion.