I also have back issues and I was told to take an light aquafit course - they are usually offered at the YMCA or in my city community centres. I was told it is good to do as there is little pressure put on your joints as the water makes you boyant (you only weigh 10% ie: your weight 150 lbs on land in water 15 lbs). It has helped me in my mobility and has increased my muscle strength. Good luck in finding somethingthat works for you.
Female, 47 yrs of age
Start weight: 249 lbs ( 3/9/12)
Mini goal: 215 (7/30/12)
Goal weight: 148-152lbs (12/30/12)
Current weight: 249 lbs
Loss to date: 0 lbs
Inches lost to date: 0