Transfer case: switch to both 4H and 4L to see if it works
Axle: Turn the front wheel to left/right most position, take a look of the axle to see if it is
in good condition
Lock: I have no idea on how to check differential lock,
can you provide a little hint?
Transmission: Engage in all gear to hear the sound
Engine: check engine oil
Suspension: Push the vehcile down on all four corners,
to see how many times of bumps it takes to recover to static status.
Rust: just look under to see if there is any rust around
You suggestion/correction is highly appreciated.