Pilates will do wonders for your lower back strength and stability, which it sounds like you need here. Actually it develops strength and stability in the abdominal muscles which will support the back and basically take a lot of the load for supporting your torso off the back muscles and put it in the abdomen, where it belongs.
Also, hamstring flexibility is a key component of lower-back health as well, so yoga poses like downward-dog and forward fold (standing or seated) are good to do on a daily basis.
You will laugh, or crinkle up your nose, but honestly ballet is one of the best types of exercise out there to protect the back and keep it strong and supple for many years. See if there are classes for adult beginners in your area (you'd be surprised how many are out there, actually). Alternatively, there are some good videos that can help you: New York City Ballet workout being a good one in terms of the exercises (but sort of lame in terms of production value). Balletone is another line of ballet-based exercise videos that is popular, although I personally can't recommend them one way or the other because I've never tried them.
There should most definitely be Pilates classes in your area though, because they're becoming ubiquitous, and I'd recommend starting there if ballet isn't really your bag.