You need to get in there yourself and see where it's leaking from. At least narrow down the genral vicinity of the leak. It could be anything. Warm it up and keep pouring water in. These trucks are famous for cracked thermostat housings. I recently thought mine was bad, but it turned out to be a pinhole leak in the bypass hose between the thermostat housing and the water pump. I changed the hose anyway along with replacing the bad hose. My 2002 has 171K on it already so old coolant system parts aren't something to skimp on.
The radiators have plastic tanks. On any vehicle plastic tanks crack at some point. Full aluminum radiators are on eBay, but the only ones I've seen are made in China (I have one in mine).
The heater hose going into the water pump can rupture, too, especially if it's old.
