90 Geo Tracker 4x4. 4x4 doesn't engage

I know that there are many thing that it could be. But just need a good place to start looking. 4wd high doesn't engage at all. But when I put it in 4wd low it uses the low gears but only the rear wheel will turn. Any info on what I need to look at would be greatly appreciated. BTW I just bought this thing and I have no clue where to start.

this probally the problem they are bad to do it the hubs are worn out and the replacement factory are no better go to a 4 wd center and order a set of warn hubs and no more problems ever promise