How do you make all wheel drive into a 4 wheel drive?

Answer 1

Depends on what your vehicle is. On something like an Oldsmobile Bravada - which uses a transfer case - you simply replace it with an NP231 transfer case. For something like a Subaru, which uses a differential attached to the transmission, it gets much more complicated, and would ultimately require either extensive modification to the drivetrain, or mounting the car body onto a compatible 4x4 chassis.