It's been about a month or so now and I have averaged 15.5-16.5 town/country driving. I have seen 18.5-19 on the highway at 68-72 mph
I can't contribute all of this to the "2hi mod" because I have been trying to drive with a much lighter foot I do not let the tach go past 2500 rpm and when I feel a shift coming I lightly let off the throttle, it seems to help a lot but I still love knowing that I am only in 2 hi and not constantly slipping clutches in the transfer case
Since you have a 2000 expy if you disconnect power from the A4wd it just keeps the transfer case from engaging the clutch as mentioned. Generally the vehicle is in 2wd mode unless slip is sensed via the gem module/anti lock brake sensors. It should make no appreciable difference in fuel milage as the complete front drivetrain is still spinning down to inside the transfer case because the wheels are spinning. Its just cannot apply power to the front drivetrain if the rear wheels were to spin.
My guess is its your driving style. I have pulled the fuse disabling my A4wd and noticed no difference in fuel milage myself.
Unfortunately we lack the center axle disconnect of the earlier expys and IWE's of the later ones that would keep the front drivetrain from spinning creating additional drag.