Wendy, I have the same exact situation as you. My BMI is 45, I'm 26, and my doctor has blamed irregular periods, back pain, etc on weight as well. But they offer no helpful suggestions on how to lose weight aside from "eat less, exercise more." Well if it was that easy, I'd have done it by now!
I can only stand/walk for 15 minutes without pain too. If I walk for more, I can do it, but it hurts. I went to the mall for a few hours with friends, and my back was throbbing by the end. Very terrible.
I was told that yoga and pilates can help out with back pain with my weight, so I'm going to do that. Also, I plan to stand up and do a full body stretch every hour when I'm at my computer. I think part of my problem is sitting too long. While I have very nice posture, I think this is a big cause of the back pain.
But I am planning on trying yoga to see if that will help. I'm going to get a dvd and not go to a class though.

I would be too stressed out to go to a class.
Anyway Wendy, I hope that you keep us informed about your back pain, and good luck with it!