I filled my radiator with water and it is pouring right back out of the rear of engine as fast as i fill. It seems to be near or behind the transmission.
How can I find the source of the leak?
There are three places I can think the water would be flowing out of from the back area of the engine. The engine in your Yukon should be the GenI Small Block Chevrolet (SBC) engine (I believe they started putting LSx motors in them in 1999). The SBC has water passages through the intake manifold at the front and back of the engine. If one of these passages had an issue with the intake manifold gasket, it would leak off the back of the engine.
The second place to look for is in the side of the block where the freeze plugs are at. If one of these has corroded through, it would be dumping coolant like nobody's business. (The block in the picture doesn't have plugs in it yet, but there'll be some there before it gets used!)
The last place, and most likely is another freeze plug, but at the back of the engine. This would be inside of the bell housing of the transmission. You'd see it coming out the bottom of the torque converter dust cover. (The plug in the center of the picture is there to plug up the hole where the cam goes; no coolant there.)