Find a wrecked body 4x4. Unbolt your trucks body from your 2wd frame put on 4x4 frame... Seriously it is more expensive to convert than to sell and buy a 4x4 to start.
But here is a list of things needed from the top of my head.
Rear output shaft
Front half shafts
Front wheel hubs
Some other suspension bits.
You might also need the 4wd transmission that goes to the 4wd transfer case and then to the front and rear differentials.
So yeah literally over 10k in parts.
Would be cheaper to find a 4x4 wrecked to pull needed parts but like I was saying at the top you would need almost the whole rolling frame minus engine.