I have always gotten in the low 20s mpg with my truck. This last month it has snowed a ton in Colorado and I haven't had the front end disengaged at all. I have gotten the worst mileage ever in the 10 years that I've had this truck at 17-18mpg. The low temps and resultant high gear oil viscosity mixed with stop-and-go really killed my mileage. Of course that includes the oxygenated winter gasoline and denser cold air. But I have never gotten under 20mpg before even in past winters (but I only had the hubs locked maybe 7 days out of the whole winter).
I had been thinking that maybe something was wrong with the engine but I just got 24 mph (highway at around 7k feet) this weekend with front disengaged which is what I get in the summer.
I guess cold weather mixed with stop-and-go and having that front end turning all the time really can affect mileage.