Those 4.0 straight 6 engines will run forever, so your's seems to be the exception to the norm.
If you have the skill and time, I'd suggest rebuilding the current engine. There has to be something seriously wrong with the engine if it is burning that much oil and fouling plugs like that. I had a 1990 Cherokee Limited and a 1994 Grand Cherokee Laredo with the 4.0 straight 6. Both engines ran over 100,000 miles without buring any oil, always have used full synthetic in them, but the value cover gaskets and the oil cooler lines always seemed to seep oil.
