What's wrong with your back?
I stretch my hamstrings sitting on the floor with one leg stretched out in front of me, and the other leg tucked up so my knee is out to the side and the sole of my foot is against the inside of my other leg. Then I reach my same side hand towards my toes on the stretched out leg. Change legs and repeat.
I'm not sure if this would be any easier on your back, though.
Other leg stretches shouldn't involve your back at all, so I'm assuming that's the one you're having problems with.