There is a site that is dedicated to NAPCO 4X4 units that were available as retrofit kits before the factories produced 4X4 trucks. I'm more familiar with Task force era trucks, but I believe there were kits for your era trucks as well. Try googling NAPCO 4X4. That would be the most direct way to do it. Ive seen Advance Design trucks with Blazer or Jeep truck drivetrains or even modified frames under them. Thats a lot more work and takes more engineering to make a truck that works well. Good luck.
LJ
