i didn't have a good handle on a bunch of back/knee issues till a doc did a series of x-rays of my hips to my feet to measure skeletal leg length discrepancy of 6mm. functional leg length discrepancy is totally different from skeletal, and are treated differently.

i'd recommend the same thing, as it will help rule out or confirm an important possible causal issue. a 5mm shim in my walking shoes, and a cleat shim for my cycling shoes has made a HUGE difference for me.