Wow! I've had SI problems for several years after falling down oak hardwood steps on my butt and back. It was so painful to sit down that I worked standing up at my desk for 1.5 years. Visit and visit to the PT's and chiropractors and icing it down. After about 4 years I have it mostly under control, but it will flare up if I don't sit up straight or sit in a soft chair.

I've been doing Feldenkrais lessons for a few months now and it is also helping me. Another funny thing that helps me when my back gets tight is to get on my hands and knees and totally relax my stomach muscles.... kind of like those yoga poses where you make your back convex and concave.

But no one has ever explained it like Knot did. That was absolutely awesome.

sfa --- I really hope you find the clues from your body about how to get past this.