OK, as larabell says, as US citizens, you are both subject to US taxes for the rest of your lives, no matter where you live in the world. And chances are, you'll wind up paying tax to the US on your social security and government pension - however the details of this depend on the tax treaties between the US and Japan.

One small correction to what larabell said, however. You are not eligible for the overseas earned income exclusion for your pensions. The exclusion applies only to earned income - i.e. salary earned while resident overseas.

Chances are you'll also have to declare your pensions to the Japanese tax authorities, but to the extent that you pay taxes to the US, you'll have some mechanism for crediting those payments against whatever your Japanese tax liability would have been.

Can't help you with the Japanese side of things, but for the US taxes you can download Publication 54 from the IRS website. There is also a publication that explains taxation of social security benefits in great benefit that might be of interest to you.

When you get to Japan, you can get tax help through the US Embassy. Chances are there is an IRS office in the Embassy. You can also contact one of the US expat groups in Japan and they should be able to offer some assistance, and possibly even suggest a US tax advisor (usually one of the members).
Cheers,
Bev