Vicki Sims is a physical therapist who has done a lot of work focusing on the SI joint.
She runs Gainesville Physical Therapy in Gainesville, GA, where she consults patients.
She also works with surgeons who have been trained in SI joint fixation, and some patient testimonials say she comes to the hospital to make sure patients are aligned prior to their fixation surgeries.
Here are a few great places to start:
She is the author of The Secret Cause of Low Back Pain.