Well I asked a trainer about the stair stepper today, and he said they took it out of the workout room to fix it!
I generally avoid the treadmill, but I learned today, the steeper I set the treadmill, and the slower I walk, the less impact it has on my back. It's not as good as the stair stepper, but at least it's something.
I also walk my dog on the beach when the weather is OK. Walking on the sand is a great workout for someone with a bad back like me. The sand softens the impact, and walking in the sand is also a great workout. Walking on concrete is painful, but I can walk on the beach for miles. I'm literally dripping with sweat and my calves are burning after I walk on the beach. It's also great for my dog too of course. The beaches in WA state aren't that populated so you don't have to use a leash, so there's usually other dogs she can play with