You could also check with a chiropractor who is trained for maternity and babies. I saw a chiro for the whole second half of my first pregnancy and it was a life saver. I'd go in with a waddle and sacrum pain and walk out feeling great.
I second the chiro recommendation! Total life safer!
Try doing some prenatal yoga. It's not hard it's just simple stretching. They have DVDs at Walmart. You'll get relief almost as soon as ur done w the vid. It helped me alot with my 1st baby.
I had this problem. I started having pain at 20 weeks. There's not a whole lot you can do. Get a pillow that has a cut out where your tailbone sits, amazon has them. That will help a lot. Avoid hard surfaces, don't do things that make you lean back onto it. If you don't have or can't afford a pillow you can sit on a rolled up towel in the shape of a u to take the pressure off. You can probably find a YouTube on how to do it. Also talk to your Dr.