When buying a used 4x4 pickup, make a short list of models for consideration, then visit a dealership, and physically inspect the vehicles on the list. The most important areas to inspect include the 4x4 system, tires, brakes, underbelly and suspension. It's best to have a mechanic inspect any vehicle in which you are seriously interested.
Make sure the 4x4 system works properly. If an owner's manual is available, switch between the various 4x4 modes detailed in the owner's manual, ensuring each mode engages and disengages as instructed. Listen for any abnormal sounds or vibrations. If there aren't any, that's a good sign that the 4x4 system works properly.
Look at the underbelly for wet-looking marks as these may indicate leaks. Check the frame and suspension for cracks, damage and corrosion. Check the tires for uneven wear.
If you find any problems with the vehicle during physical inspection, have the seller address the problems before proceeding. If the truck passes your inspection, take the vehicle out for test drive. The truck should not have any starting problems and should operate smoothly on the road, without any excessive noise, vibration and harshness. Make sure there are no significant delays in the engagement of the transmission and the 4x4 system. If everything about the used 4x4 truck checks out, negotiate a good price for the vehicle.