SI / back pain

blimey mate sounds like serious overload. From your history and the types of rehab you've had I would say go back to basics. forget loading, forget taking the spine through range, you need to work on your core stabiliser muscles - but you need to do this correctly. I've treated so many "chronic backs" in exactly the same situation, young, fit and generally healthy - but the worst core stability going. I would say go back and speak to your PT (don't know if they're the same as over here - uk) or go and look up core stability on the internet - this site has some good info. Don't look at any exercise using the ball or anything that involves high loads - really basic, you've gotta undo everything that has happend to your body - undo all the work you've had done to you then start from scratch. Let me know how it goes - be interesting to see