I have one molly mule that the riding saddle and pack saddle consistently shift over to the left side. This occurs no matter who is riding her. I have checked a lot of things and even the vet can't see anything wrong.

Could this be a stride problem where she is taking longer steps on one side vs the other or? I have even measured her stride when walking and trotting and can't see any difference.

If so, what should I look for, and what steps can be taken to correct it? Are there any publications/books on the subject you could recommend. I do all my own shoeing and have done so for about 35 years. Around here, there are not any farriers with a lot of experience in shoeing mules. Most of them want to shoe them to look like a horse.

I don't really know what to recommend to you. Even if there is a shorter leg, I doubt that it would cause the problem. If you stand elevated several feet behind a horse or mule, you can look down the back and see symmetry, (or lack of) in the withers. If there is some, I would try fixing it with a pad system.