The 2wd frame needs extensive modification to meet the requirements of a 4wd set-up. Then the front suspension (ALL of it) has to be replaced by 4wd suspension. The front differential gear ratio must match the rear differential gear ratio. The steering system has be to from a 4wd truck (integrated into the suspension and front axle assembly). The transmission is different in a 4wd truck to accommodate a transfer case all of which your 2wd wiring harness and computer and not set up for. Then there is a laundry list of minor details (plumbing, vacuum lines, transfer case shifter install, cross member, drive shafts, u-joints, etc.).
The long and short of it is... It is far more economical to sell the 2wd and buy a 4wd truck than it is to do the extremely complicated and time consuming conversion. Not trying to rain on your parade, just want you to be fully informed of the task you are considering.