they are good trucks don't let this post deter you from going and checking that truck out.
exhaust.. even the perfect ones leak a little.
transmission if it's sloppy and hard for the current owner to make the shifts, you don't want the truck because you will have to replace the transmission.
Drive shaft and u-joints.
if it wiggles or is just plain sloppy, don't buy the truck because when the ujoints go out, they add wear to the transmission tail and the rear end.
Check the brake rotors for grooves.. if they have grooves ground into them but the truck stops without screeching of grinding noises, the seller replaced the brake pads but not the brake rotors which means you will have to replace the brake rotors and pads. (groves come from brake pad rivets after the old brake pads wear out. Then the rotors chew up the new pads which causes them to fail prematurely)
tires... if they are not worn evenly, there could be a host of repairs needed