Changing axle/s, transmission, transfer case (if equipped) fluids.
Repacking of bearings.
Greasing of underchassis components (if equipped with grease nipples).

Check for torn rubber gaskets/boots (e.g. torn axle boots). Replace as necessary.
Check undercoat for any chipped/peeling areas. Correct as necessary.
Regular underwash.

Check owner's manual for exact maintenance schedule of above mentioned items.