We have a bit of a chicken and egg scenario re SI. Ron had a slightly unlevel tuber sacrale area when I bought him (april 2007), which caused him no problems for at least a year. His canter was always poor, canter transitions poor, had a great jump but was lazy behind over smaller jumps, usually knocking them down unless bigger than 3 foot. He then started bucking when excited/stressed (april 2008), which I put down to spring grass and him being 4/5. In the autumn, he had a couple of crashing falls in the field, and was then 3 or 4/10ths lame on left hind. Presented to vets like a fractured pelvis, which necessitated an urgent trip for a bone scan. Bone scan showed SI issues. He was medicated twice in to the joint with corticosteroids, came sound, and then started work.
Then went lame on both hinds (and one fore). Again 3-4ths lame both hinds. PSD then diagnosed, and surgery recommended. Neurectomy and fasciotomy only it went a bit wrong as the wounds broke down, so he couldn't do the gentle exercise, so adhesions have formed. Lots of physio later, as his back was in a lot of pain.
He is now in work, is intermittently very mildly lame on all 4 legs (!) - thought to be mechanical behind (and isn't noticeabe to my excellent instructor) and they have no idea what's wrong in front (possibly nothing - referred lameness). I *think* we have come out of the other side of the SI pain. He works better now than he ever did, doesn't buck, and the canter is a totally different, *great* canter, compared to what he was like before all this. So I wonder whether he had something wrong all along. I now do most work long and low, some time in deep and round, and have started to ask for more "up" work, which he is finding fairly easy, thankfully. Hopefully this is all strengthening his back and SI region. I am hopeful he will be a hack/low-level dressage horse now.