How about front squats on a smith machine or regular squats on a smith. At least it would take a lot of pressure off of your back, and I'm sure you already belong to a gym, so it wouldn't cost you any more cash.
I tried front squats and I really hate them but I will give them a go on the smith... Only problem the gym I go to the smith suxs bad .They only have one and nobody uses it he has 5 power racks thou. I figured out amazon has the belts i might order it and try it. Free returns on amazon.