There are many different kinds of T18's if you are lucky enough to find one out of a cj and it has the adapter to bolt it to the model 20 transfer case it will bolt right in, however your driveshaft lengths will change. You can get a T18 out of a jeep truck as well but it will need a short input shaft put on it. Ford T18's will also work with some modifications.
This guy knows all there is to know about T18's
http://www.partsmike.com/tech-help/id-t18/