The obvious stuff:
Crawl under it and look for damage, leaks, check the frame in the standard areas (steering box, cross members etc) for cracks, pivot arm and other bushings.
Open the hood and look for leaks, hack jobs, vacuum lines, wiring issues etc., check the fluids for smell and color.
Drive it and feel for pull, listen for bad sounds, make sure it doesnt have death wobble issues, that the shocks arent shot, that the brakes dont pull, that the tranny shifts properly and that it does not slip.
Shift it into 4 wheel drive (not on pavement) and drive it that way. Do some figure 8s and see how it does. Does the 4 wheel drive engage. Does it feel like its in 4 wheel drive? Put it in reverse and drive it that way as well, and do the same in 4 lo.
Take some wire and pull codes. It might seem weird, but there are a lot of expensive sensors that you can have codes for that you will want to know about before you buy it. If it passes (1-1-1) cool. If not, you have a better foundation to make your decision with.
Ask about maintanence records....