I just finished 2 months of physical therapy for a strained back and degenerated disc that started to affect my sciatic nerve with pain all the way down my right leg. The safest exercises are going to be stretching. Using a band, my pt started me with calves & hamstrings, then strength training for the thighs and core strengthening (abs) exercises that help to take some of the pressue off of your back. I used a heating pad every day. And sometime ice if I was really swollen & sore.
What she told me was the thing with a back injury is those muscles that are injured can tighten and pull other muscles and joints like your hips out of wack.The best thing you can do is stretching which will help all your other muscles loosen up and move freely giving your back a better chance to heal.
Check this out: http://www.bigbackpain.com/post_injury.html
Best of luck and if you have any questions, just pm me. Feel better