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Overheating starts with water loss. The 3.9 and 4.2 engines were basically an over-bored 3.5 casting with 4mm extra on the diameter of the liners. This caused a reduced thickness of aluminium between the water jacket and the cylinder bore. The subsequent water loss problem normally starts off as just a water light that appears once a month or so, then once a week, until it becomes a permanent feature. The normal unsuspecting owner will have by this time paid for heads to be skimmed and gaskets to be changed, and they will have spent a lot of money already. So, although the engine functions fine, it is a permanent worry leading towards a total engine failure.
Many might be excused for thinking that the overheating is caused by running the engine in a hot climate, or with a radiator problem, or even insufficient cooling fans or oil cooler, but we can assure you this is not normally the case.
The true & main reason these engines run very hot is due to the fuel/air ratio, or fuel mixture, that is controlled by the engine EFi computer (ECU Chip) from the factory. This was designed to run very lean through the mid range to make altitude driving or mid range emissions (tested in some countries) less of a problem [...]

4.0 & 4.6 (new shape) suffer a similar but normally less dramatic problem although these engines are much stronger. The same air/fuel ratio problem will arise with low octane fuels, you will not normally suffer cracked blocks, but you will cause the liner to shift from its seat or cause sticky valve guides (partial seizure), and the solution is much the same as the 3.9 & 4.2 above, an ECU re-chip will sort it out
All cracked engines (blocks) are not normally repairable due to the fact that it will crack behind other liners, even if you could effectively repair the one at fault.
If you are undecided about where the problem actually is, then the next job would be to remove the heads, and if you see nothing suspect the worst (the surest way to tell if your engine is suffering from this problem is the fact that you can find nothing obviously wrong). You could have the heads & block pressure tested if you are still not convinced).