Originally Posted by wydopnthrtl
Well.. there are only two options left. Evaporation via a small leak somewhere that's not enough to drip/puddle. OR it's being burned in the engine. Blown headgasket, cracked head/block, ect..
First I'd pressure test the cooling system and look the radiator over for whitness stains of a leak. If you don't have a pressure guage/cap.. run it for a 1/2 hour driving and then park it in a quiet garage and carefully watch listen for a good 10 minutes.
Second I'd pull every single spark plug and look for any that are not the same colors as the rest. If you do find that.. you know it's being burned because of an internal leak.
Good advice. And a compression test could be useful, as well.
Something to keep in mind too: a failed lower intake gasket can also lead to coolant sipping on the 4.0L OHV, particularly the earlier ones.