I too have a bad back and wanted to carry my daughter.
I love the long wraps, but they can be difficult to wrap in public if you have to take baby out, and can get pretty hot.
My other fav is the Ergo Baby. It's a bit pricey... but you can wear baby front or back, or on the side. It spreads the weight out very nicely and it is comfy.
You can get it through:
www.myfavoritebabycarrier.com with a 90 day money back gaurantee!!! Seriously... 3 months. So, try it for 3 months and if you don't like it.. .send it back for a refund. You really can't beat that deal. No one else gives you that. By the time I found Ergo Baby, I had gone through 4 other slings. I thought 'what do I have to loose!' I bought it and loved it. My daughter also likes it. Check out the website...there are a ton of features that they do a better job of explaining then I could.
Oh... and the weight limit is pretty high... so you can use it a long time.