What's My Mobility Issue?

When I sit down into a full squat, my left glute hangs noticeably lower than my right. What’s causing this and what mobility drills should I perform to fix it?

Possibly some form of pelvic tilt? Have a read of the following articles.

http://www.T-Nation.com/readArticle.do?id=1508256

http://www.T-Nation.com/readArticle.do?id=459846

http://www.T-Nation.com/readArticle.do?id=990092