My mare was kicked in the hip by a much larger, shod horse, and started to behave much the same as you're describing. At first I was convinced it was a fractured pelvis, but the vet came, checked, and cleared her on that. After watching her muscle on that side waste, and then she began to have a complete personality change, I brought out a chiro on a hope and a prayer. Turned out the kick pinched a nerve in her back, which ended up restricting blood flow to that hindquarter, made her move unevenly, etc etc etc. It's crazy how everything ties in together! Anyways, after just a couple visits, she began to improve at lightning speed. We had to do a bunch of work on helping her learn where to place her hind feet, and lots of long and low to help her stretch out all her muscles, but we're pretty much back to normal now. Good luck, I hope your girl turns around for you.