When looking for a jeep you should of course check the obvious signs of abuse, rust, leaks, body damage, torn up interior, etc... Then look under the hood and check the fluids, a well maintained jeep will always have its fluids up to par. Underneath check things like u-joint and driveshaft play, check the pumpkin for leaks or anything that may be out of ordinary. If you find a lifted jeep these things become more of a common issue. If it is lifted make sure you ask what kind of lift, if it was regeared, if anything was done to help the driveshaft angle (wether it be a mml, sye kit, cv driveshaft or transfer case drop) and what kind of rearend it has. Oh and make sure the 4wd works
Im sure someone will give you more info as im sure i missed something
Good luck and happy hunting!