Not sure if the 4X4's had the rear air shocks but if they did make sure the compressor is not cycling a lot. Can be very costly to replace. If you are buying from a dealership you might see if they will replace spark plugs, fuel filter, and transmission service. Also, ask the dealer for the door keypad code if equiped.
crank the wheel all of the way whil driving and listen for popping (your listening for sway bar ends). watch the speed up/down in the windows (watching for regulators). make sure the TPMS light is not on (looking for bad stems/bands). run the ac thru the different modes - panel, floor etc (listening for blend door problems); check rear as well. seat heaters if equipped. check for tailgate rust. will probably need a fuel filter now also if the previous owner did not do it. other than that-enjoy. Mine had a lot of the above issues, but the price reflected it. All fixed now and absolutely loving it - not even sure if I will go back to a truck again now. Hope this helps, Mike.