How do you convert a single rear wheel truck to a dual rear wheel truck?

First, you'd need to find a live axle for the front, and ensure the gear ratiOS in both differentials matched up. You'd have to do a complete front axle swap, and might also need to add a leaf spring or two to the front, as 2WD vehicles typically sit much lower than 4WD vehicles. If you have to raise it high enough, new struts and a new Pitman arm will also be in order.

The transmission will likely have to be replaced, or you'll have to replace the casing in order to have a mounting surface for the transfer case. Then you'll need to replace your driveshaft, as the one in a 2wd truck will be longer than what you need for a 4WD drive truck with a transfer case attached to the back of the transmission. You'll also need a crossmember for the transfer case to sit on, plus a driveshaft for the front axle.