When you find one you like, get under it. What axles? Is anything banged up? How bad is the rust down there?
Now give it a cruise. Since you're looking at automatics, be sure to get the engine warm, and get it to 65-70mph. Make sure the shifts are smooth. A little firmness it ok, but they shouldn't slip. Minor slipping with a cold engine is normal from 1st into 2nd but that should quit when the engine warms up. Make sure you engine idle is low and no weird noises are produced.
Before you get out, be sure to engage 4hi and 4lo, and cycle drive, 1st, 2nd, reverse, and OD off through each. A little force is normal when selecting 4x4, but nothing crazy. 4lo is sometimes tempermental. Normal TC behavior. Simply roll 1-3mph forward or reverse, transmission in neutral, then engage 4lo.
A vehicle that old, watch for corrosion under the hood, mainly with the electrical systems. Look at the oil, transmission, and other fluids. Everything should look nice.
Now if everything checks out good mechanically, you like the price and mileage, start signing papers. BUT before you're finished, take one last look at the Jeep. It's been sitting for around 30 minutes at this point after a rather rough little drive at highway speed and gear shifting. Look under it. Anything leaking now? Check both differentials. See any fluid? If its still ok, finish up signing.
Once you're in your new jeep, proceed to drive to the nearest tire shop and get yourself some 33" sand tires and some lift. Then go to town!