The screech is probably from the serpentine belt that drives, among other things, the radiator fan and the water pump, which of course are important in keeping that jeep from over heating. The belt may be loose, they stretch over time, the belt may be 'glazed over' from rubbing against the pulleys over time, you may be able to solve the whole problem just by tightening or replacing the belt. MY bet and my hope, because of the financial aspects, is that will cure your problem. ( Is it still the original belt? That's eight years and time to replace it anyway.) It may be that you need a real good flushing of the cooling system, make sure the flush includes the heating system. Something is preventing the water from making its complete circuit and cooling the engine. A loose or glazed belt will slip over the pulleys and cause a high pitched screech that increases in volume and pitch if you give the engine a quick rev. If the engine is idling, for example when you 'pull over,' the belt might not be moving fast enough to screech, or may actually be getting a better grip, try a slow acceleration and see if the screech doesn't start until a driving speed is reached or you give it a quick acceleration. One or more of the idling pulleys or a driven device may not be turning freely due to wear, a simple lack of oil, or a mechanical failure, causing the belt to slip. A slipping belt might not be turning the radiator fan or water pump fast enough, causing the over heating. Airconditioner belts are especially prone to slip since turning on the compressor causes a sudden heavy load and the belt slips. The puzzlement here is why it only happens when the heater is turned on, usually turning on the heater does not add any additional load to the belt, and actually should reduce it. Some defroster settings do turn on the airconditioner as the AC dries the air as it passes over the coils. I do not know if your Jeep AC runs off the same belt or has a separate one, but try turning on the AC with the heater off and see what happens. In fact try all the combinations of AC, heat, defrost, and engine speeds (with quick and gradual accelerations) that you can. Checking will be easier with two people, one to be looking under the hood while one revs and changes heater settings. Park the car, put it in Park or neutral, set the parking brake, even chock the wheels. Before you start it take a good look at the belt(s), trace it all the way over, under, and thru the various pulleys and wheels, locate the AC compressor and the water pump. Push against the belt where it makes a long span between pulleys and just see if it feels to loose or too stiff and brittle, see if you can turn any of the pulleys by hand, you shouldn't be able to, but also feel if any of them seem to be 'wobbly.' Now start the car, and go thru the various on and off fast and slow procedures while the observer watches. Just watch, do NOT be putting your hands in there, in fact put your hands behind your back. A work light that you can hang or clamp some where is better than a flashlight that you might stick sumers it shunt go. If you're lucky the observer might be able to tell exactly where the screech is coming from. The ol' timers will often use a long screw driver or other rod to localize a sound by putting one end against the suspected, *non-moving* part and the other end against their ear or jaw, but then most of them are old timer men with short hair and no jewelry or other dangly things that could get caught in a pulley and decapitate them. Sometimes it can be hard to tell if a pulley is spinning, resist the urge to reach down and feel it, with the engine OFF use a piece of chalk or a Marks-A-Lot ( I'm an ol' timer maseff) and draw a line from the center to the edge. If the line don't move, or even if you can just barely make it out spinning, something is wrong at that pulley or the belt is too loose. Like I said, I'm puzzled as to why turning on the heater would cause screeching and over heating, in fact turning the heater on is a recommended emergency procedure to alleviate over heating. The heater gets its heat from the same water that cools your engine. Somewhere under your dash is a 'heater core' which is nothing more than a miniature radiator just like the one under the hood. When you turn on the heat, usually with a lever or 'sliding' switch, a valve, usually located right on the water pump housing, is opened.( There may be cables and/or vacuum lines between the actual switch and the valve.) The water pump will be at the belt end of the engine, towards the top, with a larger hose going off to the radiator and two smaller hoses going off thru the firewall into the under dash area, one of the smaller hoses, coming from the heater, will connect right to the pump housing, the other one may come off just about any ol' where. Some of the hot water from the engine is shunted to the core where the fan blows air across it and then the water goes back to the pump, usually the pump 'sucks' the water thru the core rather than forces it, in case the core has a leak, that'll be the hose connected at the pump. Even if the core was completely clogged, water would just go on around thru the regular cooling path, you just wouldn't get any heat. But a clogged core could cause the pump to try to suck the clog on thru which maybe is causing some sort of cavitating or a gap in the water trying to cool your engine. It's also possible, but improbable, that there is some kind of foreign matter in the heating line that actually gets sucked into the water pump and prevents it from turning. When you turn off the heater the Bernoulli effect at the other end pulls it back out of the pump. If replacing the belt and adjusting the tension don't help, PAY for a thorough high pressure flush, ( after eight years that's a good idea anyway.) after that an annual do--it-yourself flush is a good idea. I got wordy, my trade mark, I am sitting here trying to think of anything I left out. I can't think of any way the thermostat could be involved. And I got into more detail than needed just because I know how to do some of that stuff and hate to pay sumone to do stuff that I know how to do even if I ain't got the time. It is a good idea to try some simple tests, which really should take you less time to do than to read this ramble, and defiantly less than it took me to type, before going to the pros, and it is best, if you got to hire it done, to just tell the pros what is happening, in this case just quote your question, instead of telling them that you suspect the water pump, cause then they will just replace the pump since, according to them afterwards, that is what you said was wrong. BTW I am a ol' timer, some of my manuals actually warn me that a necktie is one of them dangly things that ya don't wanna get caught atween the belt and a pulley. If I concentrate I can vaguely remember a bunch a guys rushing to my car when I pulled in to buy a dollar's worth of Ethyl, and they was all wearing ties as part of their uniforms. I dunno if my memory is vague due to the passing of time or because of breathing the fumes of self service which came along about the 60's which was a mind altering decade in many ways. And it's near summer time now and you're gonna put this off till next year anyway. But if sumppin is wrong with your cooling system its gonna blow when it gets way hot. Best to get it all checked and fixed afore real summer when there will be a backlog at the ununiformed unnecktied mechanics.