Vicki Sims is a physical therapist who has done a lot of work focusing on the SI joint.
She is co-director of Gainesville Physical Therapy in Gainesville, GA, where she consults patients on SIJD.
She also works with surgeons who have been trained in SI joint fixation, and I have seen 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.