My father had a willys jeep, the old carburated engine was small but not efficient. You can't buy a 1/2 ton diesel truck, you have to get a 3/4 ton to get a diesel.
The current full size 4x4's all get over 20mpg (well the big three anyway, the japanese brands cant seem to figure out economy). Most of the mid sized v6 trucks/suv's all get about the same fuel economy, they certainly are not "gas guzzelers".