There werr actually a few different models made as well (1/2 ton, 3/4 ton, 2WD, 4WD, etc)
They weren't built directly by Ford, as has been mentioned, but were built by Centurion on (usually) extended cab frames. They also got lots of extra crap that the regular Broncos (which were only available as a 2 door, 4WD) never got like rear A/C, fancier interior, etc.
check out www.fourdoorbronco.com
or the forums at www.fullsizebronco.com
for more info
( I may have some facts wrong, since I just have a regular Bronco--but that'* what I've picked up so far)