I think you are comparing your 4WD Ranger to a 2WD F-150. If you buy a 4WD F-150, I can almost assure you that you will get worse gas mileage than your 4WD Ranger if you drive it the same way.
I can get anywhere from 16 to 20 mpg on the highway depending on how I drive my 2002 FX4.
I just took a 3,500 mile trip and I pretty much knew what gas mileage I was going to get by the way I was driving and the kind of terrain I was driving on. The best I got (20 mpg) was from Albuquerque, NM to Liberal, KS on I-40 and US 54 while driving at 73 mph with cruise control on. I accelerated at a moderate pace and did not floor it when passing.
The worst I got (16.5 mpg) was from Kingman, AZ to Flagstaff, AZ while driving 76 mph on I-40 and climbing a mountain and keeping the foot in the gas pedal. Overall I averaged 18 mpg and could have raised that to around 19 to 20 mpg if I decided to keep the speed to 70 or lower and did less passing on two lane highways, but I am not that patient.