Have you seriously discussed the Japan option with your husband? Is he afraid of adjusting or are those your fears? When looking at a huge decision like this, I always recommend sitting down with your significant other and putting pen to paper; list the pros and cons. Go on the internet together and really look at what life in Japan is like; it could open some interesting horizons for you. If you decide to go to Japan, then you can study for the civil service test while you are there and when you return to the States be fully prepared to pass, if that's what you still want. If, however, you can't make the change, do you have a family member or members that can help you with the money for the prep course? Or, can you take a part time job to make the money for the prep course?
Good luck and keep us updated = )