Agreed. Most likely scenario is the transmission cooler leaking into the radiator. But even if it is the extension housing seal, which as they said could be diagnosed by draining the transfer case and seeing if there is tranny fluid in the blue auto trak fluid....
Even if it is the extension seal it is not really THAT horribly bad to change it. All you would have to do it take off both driveshafts, remove electrical connections, and pull the tcase itself, then gasket changeout... Rinse and repeat...