Keep working on the overhead squats, even if it is with just a broomstick before adding weight. Do a couple of sets each day and really work on stretching the shoulders back.
Shoulder dislocates with a broomstick are another great stretch to use. Start with a VERY wide grip and slowly bring your arms (keep them straight!) back over your head until the broomstick is at the middle of your back. After doing 10-15 reps bring your grip in an inch or two and pump out 10-15 more, then bring it in further etc. Try to do at least 30 reps each day, gradually bringing the grip width in to your normal snatch grip over time. Warning, your shoulders will really feel pumped up after doing these, it's probably a good idea to do these after having a hot shower to help stretch the muscles a bit easier.