Rot at the frame, both the very end near the shackle mount, and near the front hanger for the rear spring. I hear they can rot near the steering box too, but I have not seen one.
Rot at the floor boards, in the front around the brace that goes down to the floor and behind the seat around the seat belt and roll bar. Also look underneath for rot at the braces on the floor.
Open up the air cleaner and look for either a new filter, or signs of oil. These like to have "blow by" and the breather hose should go into the top of the air cleaner from the PCV valve. (Make sure it does too!)
Look under the carpet to see if the drain plugs are all out, this could be signs of a bad water leak. Also, peek under the dash to see if there are signs of heavy corrosion that could point to a water leak as well.
Try the 4WD and see if it feels like the front axle in engaged. It should bind and buck a little with the wheel cut. If not, you may need work on the front axle disconnect.
Check all the gauges, they have ground issues and you could have some that are not working. Is it on empty? That could be a dead gauge.
As mentioned above, check for the normal "used car" stuff like leaks and condition.