Ford Repair: 2000 Explorer Sport Transfer Case, case transfer, wet ground

Question
Just purchased a 2000 explorer with a 4.0 ltr 6 cyl.
When I put it into either 4 high or 4 low and am on slippery wet ground,one or 2 tires spin, and there is alot of shaking going on. It seems as though not all 4 wheels are spinning. Transfer case? transfer case motor? or what?
Thanks
Ron

Answer
will need to checked out-several possibilities as to what the problem is from simple inexpensive to expensive-take it to reputable garage see if they are willing to check it out-if it is electronic shift some garages depending on their abilities may not want to look at it-as mechanical repairs may not be hard to diagnose some electronic ones are.