I've had a few Trackers (one I sold with over 200,000 miles) and never found a weak spot in the 4WD system.
Davepet has a good plan. Start with telling us why you believe there is no 4WD operation and we'll try to suggest things not already tried.
I'll suggest a couple of preliminary checks. The first step is to confirm that no 4WD exists. I would lift all four corners, engage the transfer case and hubs and spin a tire and see what turns (engine off!!). If everything turns, you have 4WD. If the front shaft turns, but not the front wheels, its hubs or even the front axle; it could even be that the shifter is not egaging the transfer case, but I would suspect that the shifter would feel loose if thats the case.
Another simple first step would be to pop a hub of and look inside; lots of gunk could prevent proper operation. I'm not sure if your model has auto or manual hubs, but if manual, engage and disengage while looking inside and see what happens...
Good Luck.