Have you actually been to a doctor that diagnosed this, or are you just assuming it? If you've been to the doctor, did he do anything to help/fix it?
If not, I would find a chiro/sports doctor and have him put the SI joint in it's proper place. Go to him at least a few times, while taking it easy with lifting. Eventually it should start to feel as if the problem is gone. From here, just be smart about lifting. Get your lower back, hamstrings, glutes, and abs stronger.
Back raises, reverse hypers, glute ham raises, and lots of back traction (look up dick hartzel on youtube) seemed to help me the most. It's been a couple years now since I knocked mine out of whack and I still feel it now and then, sometimes pretty bad. IF/when it does get out of hand, I'll just head back to my chiro/ART doctor and have him work on me again and start the process over...