As a fellow sufferer, I have found that pilates and yoga are good to tone up the surrounding muscles and maintain flexibility BUT you need to get some professional medical help as well, otherwise exercise might aggrevate the pain, especially if you are trying something new. I have had quite debilitating sacroiliac joint pain (where the spine meets the pelvis) and the difference after treatment from a chiropractor has been amazing; before that, I had got to the stage where I wasn't capable of any kind of exercise except swimming.
I have been doing yoga and pilates and I have found that prenatal yoga in particular made things worse because I have found instructors (trying to be helpful) have encouraged me to stretch into positions that make my pain worse afterwards. I think yoga would be great as a preventative measure, and individual stretches might be comforting, but a general class might not be the best thing for you, if you're not experienced in yoga before. IMHO pilates might be better (at least the classes I have been going to, in Discovery Bay) but best of all is to get a private instructor in either pilates or yoga, if you can afford it, who can properly focus on your particular problem. And get medical help too!
