I would go see a chiropractor; one of my legs is about 7mm longer than the other (and interestingly, my foot on the shorter leg is also smaller by about the same length). The chiropractor helps in that I have gotten a better range of motion in my hips, allowing me to function with the different sizes. Chiropractor also says that most people have varying lengths of legs, whether from bones, or from muscular structure (we have dominant legs, just like hands, which are usually slightly bigger in regards to muscular structure). I was also given orthodics last year, my chiropractor prescibed them. He had a machine that I stood on and took readings of my pressure points. I don't use them anymore, but they helped tremendously in the beginning. Walking was so easy, I never got tired out like I did before.
Your situation could be different, and obviously your difference in length is greater than mine. But a chiropractor might be able to help you, or at least direct you to someone that can. I think orthodics would help you.
Stumbled into Primal due to food allergies, and subsequent elimination of non-primal foods.
Start Gluten-Free/Soy-Free: December 2012; start weight 158lbs, Ladies size 6
Start Primal: March 2013, start weight 150lbs, Ladies size 6
Current: 132lbs, Ladies size 2
26lbs lost since cutting the crap.