I've been to two different doctors who have suggested two different reasons for my low back pain. I'm frustrated by this. Neither doctor has suggested X-rays. I'm afraid of offending my current doctor by suggesting something so obvious. Should I bring the subject up?

Absolutely. You should always talk with your doctor about your medical concerns and questions. However, you should know that X-rays are not typically very useful in helping doctors find a specific cause of low back pain. This is probably why neither doctor has suggested taking them. Regardless, get as much information as possible from your doctor about your condition, and don't be shy about explaining your concerns and frustrations.