I had back and shoulder problems, as well as headaches until I started using an exercise ball for a chair. I can also bounce and swivel on the ball when I feel like moving a little. I also can lay on it to stretch my back.
I've been using the exercise ball as an office chair for nine years, and have had to replace one for $25, and fix another with Slime TM for $10.
I highly recommend you try one. You can buy them at any store that sells sporting goods, and you can return it if it's not your thing. I also recommend you think about how often you're actually leaning back in your chair and using the back rest, because that's bad posture according to all the guides I've read. The back of a chair is only for when you're resting, and you should be walking around for your rest breaks, so it's a pointless feature for office workers.