first rule of the used vehicle, fluids -clean brake fluid, oil - translucent and doesn't smell burnt, transmission and diff same as engine oil- clean and smells fresh.
next jack up and wiggle tires from top to bottom, feeling for loose hdw and bearings, grab front and back of tires and wiggle hard- turn front tires from side to side rt and lf to check for clunking.
open rad cap when cold or cool to smell the cap and fluid, the burnt smell will forebode badness coming your way.
pedals and linkage, push brake and clutch down hard, brake should be firm and clutch should be engaging about halfway up, brake should not sink when held hard(power brakes need motor on for testing).
parking brake should bee easy to engage and not run out of travel before it stops vehicle.
check to see if everything works, turn signals lights in and outside, wipers etc., open and close all doors and windows, everything should work smoothly.
smell the interior for musty wet smell, mold is an indication of leaking weatherstripping and rust follows.
check tires dates of production, old tires = dangerous tires.
take your time, these are stout and hardy trucks and as such will be around for a long time, if you feel you need to own one then find a good mechanic you trust and enjoy!
good luck on your hunt!