I feel for you having back problems. I hate back problems! (The rest too!) I guess I'm like Goldilocks, needing something in between Papa Bear's hard bed and Mama Bear's soft bed.
When you go shopping for mattresses, spend at least an hour laying on them. I know that sounds hard to do, but you should do it anyway.
The last mattress I bought is a Kingsdowne specifically because I have these troubles, and occasionally both me and my husband have sore hips. I chose a litte softer side than him, but when he goes out of town, I always sleep on his side and have found that I wish that I had gotten the same level of firmness as him. (His is red and mine is blue - as in color coding). We could have both gotten red and it would have possibly have been better for me. (Grass is always greener, maybe? LOL!) And just so you know, THAT red is not the firmest. A really hard bed will put me in misery. Kinsgdowne has you lay down on a bed that kind of measures your body conture and you tell them where your problems are. Then the computer tells them what level seems to be for you. The man told us never to go softer than the computer recommends, but going a little firmer is okay. That's what we did.
I don't know if this helps you, and A., it's really expensive, but I figure that if it lasts me for 10 years, it is worth having a mattress that is supposed to be theraputic for our bodies.
Good luck - I KNOW how hard it is to pick the right mattress!
Dawn