SAFETY FIRST. Do this BEFORE you tart up the truck with a bunch of unneeded crap.
Inspect,
replace as needed
upper ball joints
lower ball joints
upper control arm bushings
lower control arm bushings
inner tie rod ends
outer tie rod ends
Idler arm
pitman arm
center link/drag link
steering rag joint
steering damper
front brake pads
front rotor condition
Caliper condition (Might as well disassemble, clean, inspect, lube, and re-assemble.)
front wheel bearings (check for play or noise)
CV shaft + boots
Verify receiver hitch is solidly mounted--no loose or broken bolts, frame not cracked
Verify trailer electrical connector is properly wired, and trailer brakes work as intended.
Leaf spring condition including U-bolts and spring bushings
park brake operation including park brake release and park brake pedal return
rear service brake shoes, wheel cylinders, drums, springs. Yes, this is NO FUN with a full-float axle.
rear wheel bearings
Front driveshaft including splines and U-joints
Rear driveshaft including splines (if used) and U-joints
Rubber body mounts and hardware
Tire and wheel condition, lug studs and lug nuts including spare tire and tire hanger.
Sway bar mounts and end-links
Four shock absorbers
HVAC system can blow hot air onto windshield (defroster works) Ideally, you'd have full control of blower speeds, air direction and temperature.
AFTER you have the above handled, inspect or replace
ALL filters: oil, air, PCV, transmission, charcoal canister (if serviceable). If the Air Conditioning system is to be opened-up for service, replace the desiccant pouch--which may mean replacing the accumulator.
Engine leaks--oil or coolant. Change oil and filter, flush coolant including removing and reinstalling both knock sensors to drain the block
Verify radiator pressure cap
PS hose condition--flush PS fluid PERHAPS add PS filter to return hose
Leaks at front differential (and change gear grease--about 2 1/2 qts 75-90 synthetic)
Verify 4WD axle disconnect works
Leaks at rear differential including axle seals (and change gear grease--about 2 1/2 qts 75--90 Use synthetic ONLY if you have retrofitted gaskets in place of RTV silicone at rear cover and axle flanges)
Leaks at transmission (change fluid including flush torque converter and trans cooler(s)--about 12+ qts Dex-Merc or Dexron VI)
Leaks at transfer case (change fluid--about 2 1/2 qts Dex-Merc or Dexron VI)
Charging system voltage and amperage output
Starter motor voltage and amperage input
Battery state-of-charge and load-test
Condition of all alternator/battery/starter cables/wiring harness
Condition of serpentine belt and tensioner, and each pulley on the system.
Connect scan tool, verify no codes stored, verify sensors and actuators are functioning within normal limits. If the O2 sensors have more than ~80K on them, you might as well replace all four, and certainly the front two.
You need to inspect or replace the above about ten times more than you need a trans temp gauge, and about a hundred times more than you need window tint and grille trim.
For the record, my '97 K2500 7.4L is currently out-of-service waiting on four ball joints, one tie rod end, and rear-axle/rear brake inspection. I even bought the 'special tool" J-2222-C to dick with the rear bearing adjuster.