doug,
look higher up in the hip area, had a cutting mare that would do what your describing, we got the mare after the fact, so we didnt no her complete history, chased the pain for a yr before an old time vet seen her work, and pointed out it was her hip giving against the pressure once the pressure was taken away she was fine, but it made her feet touchy, from shifting her weight on her back feet from one to the other. they used cortizone shots i believe to ease her pain