The thermostat housing leaking is a very common issue on the 4.0 sohc engine. It's well documented. You can search for more info on that particular issue on Google to find out if that's your problem or not.
They used plastic to make the housings. Super bad idea on Ford's part. It's enough of an issue that aftermarket companies have produced aluminum housings to replace the plastic ones. There are two brands available to my knowledge.
It could also be the intake leaking coolant on top of the engine. You're gonna have to do more investigating on your truck to find where it's coming from.
GB