The 33's are about 10.71%(generally) bigger than then the 33's, so your mileage/speedometer is off by 10.71%. Right now your 15mpg is actually 16.6mpg. Is your stated 15mpg highway/city/combined?, and what transmission do you have?, if an auto that might not be far from average.

Drive style also effects mpg in these heavy trucks, if you "Jack Rabbit Start", your mileage will drop a lot, yu should basically take off using light throttle. Also other than the lift how else is the truck set-up? My heavy(1000lb over stock), mod'd 3.0, lifted, large tires, safari rack/lights rig averages 15-17combined, and about 19-21 on the highway.

I would give the truck a tune-up/comp check and see if mpg's improve, as mentioned you should see north of 20mpg w/the 4 cyl.