Many possibilities for a leak. What engine do you have, 3 or 4.4? year? How many miles? That would give an indication of what could fail. From your post it sounds like the location of the leak is not obvious although I understand it leaks pretty bad. If it was the water pump or expansion tank you should be able to see where the leak is coming from when the engine is running. As X5rolls said you need to get your cooling system pressure tested to locate the leak.