I was at the local Sub'ru dealer yesterday and I described my problem to one of the techs. He said it sounds like the transfer clutches are worn and/or the solenoid controlling said clutches is malfunctioning/dead. He quoted roughly $1,000 for repairs. This would involve opening the rear of the transmission and replacing the clutch pack and solenoid.
Maybe we'll just go FWD from now on and throw some snow tires on for the winter months.