Check the hose and clamp where the lower radiator hose connects to the water pump. I had a clamp that was bad, and when the jeep was at idle I saw no leaks. Shut it down and after a minute or so the coolant started flowing - but it wasn't quite the steady stream I imagine you are talking about.
Regardless, it's a cheap place to start. Hose clamp = $2 or $3 and takes 5 minutes to swap out, if you're lucky and that's the problem. If the hose is split, $10.
Water pump will be more, but like said, don't boil the engine or you'll really be dropping some cash.
If you see Hobbes, tell him I said "Hi".