My back injury is about the same as yours. Only thing extra I have to watch is my tailbone. I broke the top joint and it's never fused and I REFUSE to let a doctor do it. I figure I've made it 5 years with this injury lost 50 pounds and managed to pass the physical fitness test for the Hotshot firefighters and I'm still walking, so I'm not fusing anything down there.
To answer your question any stretching workout can hurt any muscle group especially your back (and believe me the first few months it will), however it's all about easing into it and listening to your body. I do low impact yoga (I don't know all the technical terms) when my back hurts and when I feel like I can step it up a notch I do. Yoga, I believe is about 90% of what I do physically that allows me to continue functioning (110% of my ability to move is thanks to an amazing God). PLEASE TALK TO YOUR DOCTOR BEFORE YOU START ANYTHING!!!
Right now I'm using Biggest Loser Weightloss Yoga as I'm fairly advanced and I know how to modify to keep from hurting my back. I would talk to your doctor or local gym to see if there's a yoga studio or class offered (I started at the local Junior college). This way you have access to an instructor that you can talk to ahead of time to let them know where your injuries are and have them check your form. Getting your form right and EASING into the moves and listening to your body is the most important part of yoga. Don't expect your body to look like the wii pictures the first time or even the 100th time. It took me 3 years of yoga classes before I could do the wii fit with any accuracy...I'd also caution you that with the balance board, it changes the angles of your body so be careful!
Hope that helps. If you have any questions, please feel free to spark on by my page.