Bigblock sums it up fairly well, but I will throw my pennies into the pot. 78 and 79 Broncos will have the solid front axle for easy lift, and the majority have the 351M or 400M which has the big block tranny to make the 460 an easy swap if you want power.
If you are looking for better milage, the 94 to 96 Broncos will have a 5.0 or the 5.8 and both will have roller cams to free up some HP, and will have mass air sensors which will adjust to upgrades easier than a speed density system. All 80 through 96 broncos will have the TTB dana 44 in the front. 80 through 85 will have the 9" rear and the 86 through 96 will have the 8.8" in the rear.
4 basic bodies for the full size. 78 and 79, 80-86, 87-91 and 92-96
When you go to look at one (if its ttb) check the radius arm bushings and the ball joints