I've never driven a Jimmy, but my brother had an S10. And it didn't get anywhere close to 26 mpg. And many factory 4wd vehicles don't have locking differentials

I've never driven a Jimmy, but my brother had an S10. And it didn't get anywhere close to 26 mpg. And many factory 4wd vehicles don't have locking differentials at the front and rear, although granted the transfer case doesn't have a differential in it. So you still don't have all four wheels gripping the road. If your unlucky, you'll have one tire in the front spinning, and one in the rear.