[Big Dave]

Blister:
If you need 4x4 then you need 4x4. When 4x4 is not needed, be sure your hubs are disengaged. Automatic hubs are not as reliable as some people think.

Even when you have them disengaged, 4x4 costs your MPG in a couple of ways.

First you are dragging around a half-ton of unused machinery. That would have its effect even on my big truck operating in flat terrain. This also shows up as reduced towing capacity.

Second a 4x4 sits higher and has greater aero frontal area than a 4x2.