I'm reading a book right now about this, it's written by a physiotherapist from Netherlands named Cecile Rost, it's called Relieving Pelvic Pain During and After Pregnancy. Myself I have had chronic pelvic pain, both in SI joint and in the SP joint for nearly 3 years now. My chiro helps a lot but I'm hoping that the exercises in this book will help more too. Essentially you need to stretch teh muscles on the insides and fronts of your thighs & groin. It's going to feel counterintuitive to do this but what you're trying to do is realign the SI joint (if you're having SP joint pain your whole pelvis is likely unstable). Basics for just moving around - don't cross your legs or sit with your knees tightly together, keep space between your knees. Stand on both feet evenly, don't let yourself stand mostly on 1 leg. Ummm. what else... basic stretch is to lie on your back with the soles of your feet touching each other so your knees are spread wide - try to relax in this position until it doesn't hurt anymore, it can take 20 minutes (I do it when I'm in bed before I fall asleep, it's the only time I really get to lie down).
But anyway the book is not very expensive and I think it's going to be a good investment for me. And chiro, preferably one that works with pregnant bodies (I know you're not pg anymore but your pelvis is behaving as if it is)...
anyway tons of sympathy from me to you. It's been a long time for me because I babied my body and was afraid to move and stretch and I think I could have healed much faster if I had been doing the right exercises sooner.