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".
By todays standard a Willys Jeep isn't fuel efficient but in its day, late 30s to very early 40s, when gas was a few cents a gallon, it was pretty competitive. I'm really not suggesting anyone get a Willys Jeep. Rather I'm suggesting that the concept of the original Jeep, a small and strong all-terrain vehicle, something Jeep forgot as their design got bloated and complex, is the right idea.
Also, while American auto companies, seemingly always behind the curve, insist that diesels only show up in large vehicles you may want to look toward small diesels, from Yanmar or Kabota perhaps, to fit into an existing small vehicle or look at something like the small end of Isuzu's light and commercial truck line as they are available with a diesel. The Elf, a light-duty commercial truck with a very good reputation for durability, is a big large by my thinking but it is a truly international truck so parts are going to be available for a very long time. Find one used and rebuilding might be the way to go.