I would say look for the usual areas of rust (floor boards and wheel wells). The usual leaking around the seals on the axles, engine leaks by examing the block and the floor where the Jeep is stored. Check the color of the oil making sure there is no antifreeze in there (that could mean warped head or bad gaskets)
Look at the color of the exhaust, you are looking to see if she is buring any oil from bad valve guides or bad rings.
Was the Jeep used for plowing? If so there may be stress cracks in the front part of the frame. (inside section of frame in front)
Look to see how it rides down the road. Any 'crabing sideways' could denote an accident and a bent frame.
Try the transfer case. You can usually go by feel and sound on a transfer case. If it doesnt sound like any problems there probably arent. (torque builds up when using 4wd and especially on a hard surface so getting her out of 4wd might require reversing and some playing witht he transfer case. This as far as I know is normal.)
I know it seems a bit overboard to check all that out but after all the Jeeps and other cars I have bought I believe nothing anyone tells me!!