I'm sorry I can't be of much help since I haven't had problems with my SI joint - only my hips and knees. I too have been thru physical therapy and didn't get much relief. Many people in this forum see rheumotoid arthritis doctors - maybe you should consider that.
Have you tried aquatic therapy? Just moving around in water on my own helps me more than traditional physical therapy. Also using heat patches like crazy...
You could ask your primary about getting a cortisicoid shot directly into the SI joint - that may do the trick - if you want to avoid pain meds.
Good luck to you! Let us know if find anything that works.