Check the front and rear axles for signs of fluid which indicate seal leaks. Front dif pinion seal leaks are common, and also check where the front axle meets the dif for any sign of fluid which may indicate that the front axle has pulled out of the dif. Check the rear axle and brake rotor dust shield for signs of fluid, and also the rear pinion seal. Make sure the rear axle vent isn't clogged (if the cap spins, it is not clogged). As far as testing your 4wd, you can check if it is working by going on a snowy road, dirt road, or even on some grass. Make sure you test both 4 high and 4 low, and ensure the transfer case shifts smoothly between the two. If the truck has the offroad package, test out the e-locker as well. Give it some gas in 4x4 to ensure it hooks up well, but also drive really slow with the windows down and listen for any popping, whining, or any abnormal sounds from the difs or transfer case. If there are any fluid leaks or whining, popping sounds from the axles, I would walk away from that truck. Good luck!
2005 LE King Cab 4x4 Offroad Big Tow FFV, Purchased used on 2008/11/30.
Radiant Silver with charcoal leather interior, black Nissan OEM brush guard, 33 x12.50r17LT Hercules Trail Digger tires on OEM OR 7-spoke 17" rims, 3" Rough country Lift, Bilstein 5100s front and rear, Hawk LTS brake pads, MOOG lower ball joints, 2011 Pro4x rear end, Axle vent mod, NGK iridium spark plugs, 2 degree timing advance, Stock airbox mod with K&N filter, Magnaflow 14" SI/SO muffler on stock pipes with T304 stainless double-wall tip, Optima Yellow Top battery, OEM Rockford Fosgate audio system, Protecta chrome tool box, 6000K HID headlights, TomTom GO 920 Navigation system.