check fuses, push BOTH brake and clutch when changing the switch (4hi to 4lo specifically), check the hubs for vac leaks, replace with AVM manual hubs (I did),
BTW, how did you confirmed non-working 4wd? The front is open diff from factory, most of the time. If you engage the hubs and hand turn the lifted off the ground wheels and see the same side axle turn, it is locked in... Otherwise, you have hub problems. Check the Tech articles, they have lots of info.