Best Ford trucks for gas millage?

I am planning on buying a used Ford truck and want to know what I can do to maximize my mpg. I need a 4-door truck and highly prefer 4 wheel drive for off-roading. Any ford model truck from any year is fine, as long as it's four door, 4x4, and gets at least an average of 18 or 20 miles to the gallon. If 4x4 can be switched off and on, that would be magical.

Thanks truck experts!