I just got the book "Complete Medical Guide for Cyclists" by Andy Pruitt. It might be worthwhile to buy this book, and maybe a few others. Surely, the cost is minimal. Anyway, he does discuss the problem of different leg lengths. He indicates that a difference in leg lengths usually shows up as lower back pain. To quote the book, pg 69:

"When should I shim a cleat?
If you have a leg length inequality of 3mm or greater accompanied by back pain, you may need a shim... In cycling, if the LLI is less than 6mm, I don't use a shim. Instead, I move the cleat on the side with the short leg forward on the shoe sole 1-2mm... If the LLI is over 6mm, it requires a shim."

I got the book used on Amazon for probably $6, may as well try it!