It's probably near impossible to convert since the wiring will be missing from the 2WD model, not to mention I bet the ECU programming is different (they have different flashes).
If you could find a manual transfer case that fits (totally manual- pull a cable/lever type), it could probably be done. You're still looking at a ton of other junk- swapping or selling the truck is still the more cost effective option.
You could look at it as an excuse to do a live axle front conversion though
-Dave