hey cheeko -- I'm no expert, just kickin' ideas around...
I'd say mostly common sense stuff:
- stuff you would usually pay attn to when buying a used vehicle (check the body and interior for signs of prior damage; see if it sits level or is tilted to one side; ask if it's been in any accidents; etc.)
- make sure 4wd hi and low engage and disengage smoothly
- check to see if it has a rear locker (bunch of posts that I've read say the TRD locker (1) increases resale value and (2) is a decent locker for the price -- and not having to deal w/ the hassle of installing one later.
- check the suspension/shocks/tires for signs of abuse (are they worn out, mismatched?)
If you have AAA (or know someone who does) take the rig into a AAA diagnostic center if you can. It's not expensive, and it's even cheaper w/ AAA 'plus'. They'll check everything out -- from the frame/suspension/motor/brakes/etc. I did this when I bought a used 4Runner and it gave me total peace-of-mind.
Another thought, you could always go to a dealership and test drive a similar vehicle in "good" shape -- certified even; and compare the way the two vehicles sound/handle. I'm not sure how much you'd gain by doing this, but it might offer you a baseline for judging the truck you're about to buy.
Like I said, I'm no expert but maybe this gives you some ideas.