Is she just as unbalanced on her right lead?
If she is going in a straight line at the trot or at gait, does she travel straight or does she carry her hind end off to the right?
She looks like a horse that has pain in her left hind and refuses to bring it underneath her. Crossfiring is just easier for her. At the canter, she also looks like a horse that is carrying her hind end off to the right. It can be habit or pain.
If she does the same thing going to the right (and carries her hind end in the left lead), then it is not hurting in her left hock, stifle or hip.
Has anyone ever done flexion tests on her hind legs?
If she has no pain issues, you need to get control of her hips. You need to get her to carry her hip in toward the inside of a big circle -- first at the walk and trot (if she will trot) and finally at the canter. Crossfiring and lead problems are totally a result of a horse's hind end being in the wrong place.
I also see that she has very poor head carriage. Has anyone ever worked on getting her collected and riding her hind end up to her front end while keeping her head under control? That could also help and will be necessary for you to get any control of her hind quarters.