Romanian deadlifts or stiff leg deadlifts are an option. Here is an article explaining the difference:
http://groundupstrength.wikidot.com/...gged-deadlfits
I imagine you'll be more comfortable doing RDLs if your back bothered you while doing back extensions. If your back was bothering you from that, I would guess that you have weak spinal erectors and/or your form wasn't good.
You could try a hex bar. That will give you a neutral grip with your hands out to your sides. I'm not sure if that would help or hurt. Using hooks might be an option also in your case:
http://prowriststraps.com/inc/sdetail/7512
http://prowriststraps.com/inc/sdetail/18565