this is where i would start if it was me

1. check all diffs make sure no metal fragments are on the fill plug or drain plug.
2. Make sure fluid in diffs isnt shit anyway or if u have any at all
3. if that dont work im guessing u have auto locking hubs and just a switch if i read right...make sure the hubs are enganging at all u can tell this by lifting front end while in 4wd and turning wheels manuely. if front driveshaft spins not ur problem.
4 if it doesnt then it is
5. make sure ur shits is even hooked up for one joints could of went bad