re: Sell, Ship or Store vehicle?

Since the US government will ship it for free, you may be able to take advantage of that.

If you know someone who has a strong working knowledge of the German (or European in general) car market, see if there's a stronger demand for your vehicle there than there is here. If so, have the government ship it over then sell it there. If not, sell it here before you leave and use the money to buy something nice there that's in higher demand here then have the government ship that back so you can sell it once you get back to the States.

If that seems like a bit too much, see what the costs are to make it compliant with German emission standards. If that's going to cost too much, sell it. If not, keep it and have it shipped back and forth.

The last thing I would do would be to store a vehicle for that long (unless you or someone you know knows what they are doing). Gasoline alone has only a shelf life of about 6 months before it starts becoming gummy (meaning you would need to completely drain the tank and fuel lines before storing).