I agree, I've never heard of 4.0L head gaskets being an issue. Especially on the OHV with iron heads & iron block. However leaks, which cause low fluid levels, can lead to problems.
From my personal experience:
1. I killed a cam sensor and a clutch slave cylinder when the lower intake manifold gasket failed around the coolant crossover passage into the passenger cylinder head. It was leaking for months, but it was external, and I could top off coolant faster than it would leak it out. I didn't realize it was blowing hot steam right at the sensor body, and dribbling coolant right down the bell housing to the open inspection window and behind the clutch.
2. My heater bypass valve failed suddenly, puking out 2 gallons of coolant on a 95F day. I shut down within 200 yards and coasted to the shoulder, but I didn't wait long enough before filling the system with ice water out of the cooler from a passerby. I cracked the right cylinder head between the intake & exhaust ports in both #1 and #3 cylinders. The head gasket never failed.
3. When I rebuilt the motor, I flushed the block and the radiator. I removed a considerable amount of sediment from the rad (which in hindsight, was probably a previous owners bottle of Bars Leaks) After which the radiator end tank started to leak. A bottle of K-Seal did slow it down considerably, but I ultimately just replaced the 26yr old/300k mile radiator with a new one.
From my experience, I would be willing to throw a bottle of K-Seal at a leaking radiator tank or front water pump seal, but I wouldn't bother if the coolant leak was via head gasket. If I did use a cooling system sealer, I'd right down the date, mileage, type, and amount of sealer on the radiator core support. If one bottle doesn't fix the problem, then real repairs will be needed.
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