The one low-impact exercise that has been shown to build bone strength in the back is rowing. This article states that rowing, weight training and running all exert pressure on the bones. This article also makes the case for rowing as an effective exercise to prevent osteoporosis. So I am going to the gym, and so far am up to rowing 4,000 meters in about 25 minutes. This testimony from a woman who increased her bone density by rowing is encouraging. She achieved her gains by rowing 8,000 meters 3-4 times a week, along with yoga and Pilates.
I'd love to hear from anyone else who has had success with rowing.