transfer case stuck

It could be the switch or other things. You need to do a full inspection of the system, with a data stream scan of the trans and transfer case. A bad switch is pretty common though, so be sure to check that.

if you disconnect vac lines near the trany, it wil go into two wheel drive. pull out front plug from the front differential put wd40 make sure its clean, put it back in and lightly tap the regulatore on the side of the trany, sometimes it gets stuck, tap lightly with hammer..