Junkard axle Huh?
What to look for?
Worn out bearings, gear wear pattern, axle seals, fluid condition, brakes, axle shaft condition,spline damage, uh, a whole bunch of stuff.
I'd say if your going to pull out a junkyard axle and slap it into a truck for road use with no more info than does the gear ratio match, your friend is asking for trouble. May as well play Russian Roulette with his family.
Suppose that axle was in an accident where the axle shaft got cracked. Or it has gear teeth pieces floating around in it. Or has so many miles on it the bearing are ready to lock up. Or the seals are old and ready to start leaking grease all over the brakes. Fire or brake failure is a real possibility. And we all know what they mount next to the rear axle don't we. Can you say FLAMMIBLE?
Not a big fan of the "pick and pull and slap it in" method of car repair. My family is worth more than the money I'd save.
Just my 2cents.

CJ