Oh I SO know your pain! My SI joint has a major tendency towards having issues, not helped at all by being pg. Massaging various ligaments and muscles in the area can help a lot when you can't get to a chiro. I just google for massage for si joint and found a video with one stretch that helps me some:
http://www.youtube.com/watch?v=uuozn0i-De8&feature=fvw
There are lots of other "you might be interested in.." type videos that might be worth trying, too. There are some massage techniques that a friend or spouse could help with, too. One if essentially massaging the glute, using an elbow (so not yours!), in little circular motions, creating a pattern on the glute like:
X X
X X
X X
where each X is a pressing in and rotating of the elbow.
Another thing that I can sort of do myself is massaging the ligaments and muscles extending along the side of the leg from the hip down towards the knee. You rub, firmly, down as you pull the knee up towards you.
Both of these can be painful for me but they do a lot for releasing the tension and pulling on the SI joint, thereby helping out that pain significantly.
Also, squatting sometimes helps my SI joint.
I hope you feel better quickly!