Good idea to leave it in AWD. As gl1674 says, the transfer valve assy can freeze up. Unless of course you don't want to use the AWD any more.
How bad is your gas mileage now? Short trips of less than 20 minutes are the worse, ~15mpg. Letting the auto trans warm up before driving and accelerating slowly will help. Also syn AFT and front and rear diff syn fluids.
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