If the vet didn't see anything wrong with her, I would maybe try an equine 'chiropracter'. I haven't used one personally, but I do know quite a few people who swear by them. I've seen horses who had problems that seemed like training or equipment problems, but were actually out. Meaning they needed an adjustment or two, and they were perfectly fine after that. You never know, it could actually be something along her spine or hip that is causing her to hold her leg in that position.