Also you're fuel gauge may have been low while you were climbing or descending. Those two gallons could've been hiding in the front or rear of the tank.
I've never really considered my gas usage, I fill up before every trip and mine uses about the same amount of gas as my Wife's King Quad 750.
To give a general idea we took a 40 mile day and used a little over 1/2 the gas. A bit of every terrain, some playing in the mud and jumping dunes.
I ride almost always in 2wd high. Low is reserved for loading on the trailer, towing, attempting mud or descending steep terrain. Typically if I need low I also need 4wd. Not too often that I use low without 4wd. Sometimes descending long stretches I'll use 2wd low. Diff lock is reserved for slinging mud.
A warning about riding in 4wd all the time, if there are water bars and you are the type to press the gas instead of the brake to go over them TURN 4WD OFF. It'll be a $1,000 fix to replace your front end.
In terms of front end lubrication, everything is still spinning and coated in fluid. In fact if you think about it, technically having the front end disengaged would get things more lubed and generate less heat, allowing fluid to run off the parts slower due to a higher viscosity. Regardless disengaged = less heat = less wear which is better longevity.