Okay, you've got the sd25, which is what they put in the old Datsun trucks. You're good there. I don't know if they ever sold the diesels as a 4wd. If they did, I would hunt down a tranny/transfer combo. IF not, you might want to look into the tranny/transfer combo that International put into their diesel-powered scouts during the 70s. They ran the nissan SD33 engine in both turbo and non-turbo form. They may have the same bellhousing pattern as your sd25.
Also, when you can figure out what model # transmission you have, you can start learning what xfer cases fit on it. Just make sure you buy one that has it's outputs on the same side (ie drivers side or pass side) as the front axle diff that you're using.
You may find some of the answers you're looking for (transfer cases, bellhousing patterns, etc.) in the Patrol forum on this site. Those trucks used the diesel engines.