I would first check out your 4 wheel drive actuator which is located beneath your battery. Its like a vacuum. After you get that out, push it in and hold your finger over the hole to see if there is a leak. If there is a leak the acuator wont stay compressed which would mean you would need a new one. If it stays compressed it isnt the actuator. If the actuator is good then it could be the 4wd switch which is easy to replace. It is on top of the transmission and tells the vehicle to engage and disengage the 4wd.