It would really take a significantly bent axle to create this problem, and at that point you would either have major problems at the hub or it wouldn't even tighten up properly in the track ends. What frame are you riding and did the frame work previously? This sounds like a problem in the frame itself, such as a misaligned bottom bracket shell -- something from a bad crash or more likely, defective manufacturing. If the bottom bracket is tilted so the top of the chainring is in line but the bottom is too far out or in, it will tend to run OK when pedaling forward but will flip the chain when backpedaling. I repeat, I'd suggest you take it to a good shop (I'd recommend Bike Central) and have them check the frame alignment itself and scope for another problem. Doing it without photos or getting to try out the bike is pretty difficult and mostly a waste of your time.