The amount of money you will loose on depreciation with both those cars I would sell them. I understand that may be a wash on the GTO with the mods you would loose money on selling now so it could be a toss-up on that one. The Avalanche should go though.
Do an internet search on car storage. I have not stored a car that long but a few things come to mind. Remove the batteries completely from the car and I would probably dispose of them by giving them away or taking them to a recycling center. I would not leave batteries that long on a trickle charger unattended. Add fuel stabilizer and fill the tanks completely full. Drive the vehicle a short distance to get the stabilizer throughout the entire system. Upon return from Korea I would drain and discard the fuel. The main reason for the full tanks is to eliminate condensation from forming in the tanks. Get several desicant (sp?) bags and throw them in the interior along with some moth *****. The bags absorb moisture and the moth ***** help keep out rodents which love stored cars for homes. I would also put moth ***** in the engine compartment and trunk. Wash the vehicle and allow it to COMPLETELY dry and then put a ligth weight car cover on the car. Big debate about jacking the car and leaving it on blocks. Some say no reason with radial tires. Some say to jack car and remove the wheels so that the shocks are not fully extended which may cause corrosion on the exposed shock parts. Some say only jack the car up a little to take some of the weight off the suspension. Who knows on this one? I have also heard not to shut the doors completely ony the first catch so that the weather stripping is not fully compressed.
Neither one of these cars is a classic collectible but do have intangible value to you. Nevertheless the best thing financially for you to do is to sell them and put the money in the bank or Cd's and buy what you want when you get back.
By the way I applaud you for your service and thank you and your brothers in harms way everyday.