Oldhorselady- Your Pinto mare's canter does resemble my girls when she is running free. Have you checked into SI issues for your mare as well? My vet always says they can have 2 completely unrelated issues that are causing the same symptoms. Have you tried taking her to a university vet school?
I think I will try the University next. Hopefully they can figure it out! I'll post an update if/when they find something! The local vets are cheaper, but I'm not sure they have seen something quite like this before.