I know you don't want to be out of school for two years. But spending the time and the money, on something that is not a priority is not a good idea. Besides, a postbacc is basically a degree to help you get into medical school if your GPA is low. I know you don't want to be out of school for two years, but does it really make sense to spend money and time and effort, on something just so you have the mental satisfaction of knowing you are doing something scholastic in those two years? Because, multiple degrees do not get you into medical school. Not to mention, unless you are applying to medical school, that degree has little to no practical use. In addition to which, pursuing that degree, (which would undoubtedly be a focus on your GPA) is NOT the right decision when your focus should be getting a better MCAT score. Post bacs are for people with lower than a 3.5 man.
DO isn't bad. I don't know about the difficulty in specializing but DO is a different approach to medicine (osteopathic). Whereas, MD is allopathic.
@Goro should be able to shed some light on this matter.
Good luck to you. I'm sure you will make it. I KNOW you don't want to be out of school for the next two years by the time of matriculation, but I think it may be worth it if you get into medical school and improve the MCAT!