Cooling system mystery?

I have a 1992 GMC 3/4 ton Sierra Pick-up 4WD. I have a nasty coolant leak at the rear of the engine. The problem is that there is no way I can see exactly where it is coming from. The heater core, water pump, radiator, and all hoses are fine. The coolant only leaks when the engine is running (cold or hot) even when the system is not under pressure (radiator cap off). I have narrowed it down to somewhere between the pump and thermostat because that would be the only area under pressure. I have tried Barrs Stop leak (2 bottles) and more antifreeze than I care to buy again. It hasnt even slowed it down. There is no coolant in the oil or coming out of the exhaust. I dont think it is a frost plug unless there is one on the back of the engine. The whole engine is dry as far as I can tell. I dont know where it cold possibly be coming from. It drips mainly on the drivers side right above the starter. The higher the engine revs the faster it shoots out, judging by the cloud of steam off the exhaust manifolds. I dont have much money to fix this and I really dont want to start ripping the engine apart to locate it. If I could only see where it was coming from I could fix it. If anyone has ever heard of a leak in this location please help with your ideas. Thankyou all.

JOE