I just sold a Land Rover to get into the Yukon and head gasket leaks are a real thing that will cause your engine to overheat. Never overheat your engine or let it run low on oil, if you don't like major expenses like an engine rebuild, for instance. I had to rebuild the whole top end on the Land Rover because the head gaskets were leaking and it was a real, true PIA. I did the repair myself for under $1,000 and took me about 3-weeks but that involves doing everything properly the 1st time. It can be done faster & cheaper, especially if you know what you are doing but you run the risk of having a faulty rebuild or problems elsewhere like valve-train or vacuum leaks if you throw it together quickly & miss something or have a bad seal somewhere at the intake manifold or the like. It costs a small fortune in labor charges to hire a qualified garage to make the repairs. Also, a rule of thumb is to remanufacture the cylinder heads themselves prior to reinstalling on the fresh gaskets. Rule number 2 is do both banks, not just the 1 head which is leaking. So anyways, $2500 has always been the going rate for a V8 head gasket repair. If you shop around you should be able to find a cheaper rate but you know what they say about cheaper. Finally, you can buy a radiator pressure tester @ Harbor Frt for $60-70 last I checked. Simple to use and allows you to find what is actually leaking the coolant & where. You said the garage showed you the leak, was it under pressure at the time and was the origin of the leak pointed out to you? Anybody can find the coolant trail and put your finger on it but unless you saw that coolant leaking from exactly the interface of a cylinder head & engine block then have the next garage you go to put the coolant system under pressure and call you over to show you this leaking head gasket: they always will if it's a $1000+ repair. Don't drive the truck anymore until the coolant leak is fixed or else you will be rebuilding your engine or purchasing another 1 altogether. I have never heard of a faulty HG pronouncing itself only when the AC comp is activated but whatever; it's only going to get worse to the point where the gasket leaks under all conditions and pukes all of the coolant, frying your engine and warping the cyl-head/eng block interface making it a junk sandwich.
