Do a search for Heater Core replacement because that's what you have. I just went through the same thing in my '99 a couple weeks ago. The heater core is located behind the dashboard inside the truck. In order to change it you have to remove the entire dashboard which is a PITA. I would quit driving the truck until you get it fixed if possible because the leak in the Heater Core will eventually get worse and leak on to the carpet in the passenger side floorboard.