Thank you everyone for your thoughtful responses. I have an appointment for an annual this week, and I am going to push for a physical therapy appointment to get this figured out once and for all! I did spend some time pursuing some of the areas you suggested, and I didn't find anything to quite match what's going on, but it was still helpful. I am going to find and read the threads on measuring sitbones--that may have something to do with it, or at least be something I can rule out.