After going through the cracked 0331 head twice now (once with my neighbor's XJ), I though I would add some troubleshooting information to this thread...
The first indication of a cracked head seems to be disappearing coolant. What is left in the overflow bottle will look muddy. If you recently added coolant, DO NOT just keep adding more until you find out where it is going!
A cracked 0331 head in the 2000 or 2001 4.0 is more common than a blown head gasket. The leak is usually so slow that the oil volume does not increase, because the water evaporates and leaves the antifreeze behind to destroy the engine bearings. For this reason, the oil may not turn in to a foamy chocolate milkshake, as it does with a typical head gasket leak.
The heads all seem to crack in the same place and are pretty easy to spot. They don't crack into a combustion chamber, where the water will get into a cylinder, then drain past the rings into the oil pan. They crack on the top of the head, where the water will leak into the oil, AND the oil will leak into the water as the engine is cooling off. If you have oil in your water, that is less likely to happen with a head gasket than with a cracked head.
It is simple to check for a crack at the usuall spot, as it is right under the oil fill cap, you don't even need to pull the valve cover. Go to AZ or any other parts store with this program and borrow a pump to pressure test the radiator. Pump the cooling system to about 15 PSI and pull the oil fill cap. Look at the contour at the base of the bosses for the #4 exhaust spring and valve. You will likely see a very fine row of green bubbles here. Don't keep driving the vehicle or you will have to replace the bearings and maybe pistons too.
There was one Factory Service Bulletin about this, and their recommend fix was to apply "spray weld". I have not heard of a successful fix for that head anywhere else. I am really surprised, with the failure rate, that Chrysler was not forced to make good on all these cracked heads.
