There are steel "freeze plugs" in the sides of the engine block which are not only to relieve the pressure from freezing coolant but are necessary to fill the holes in the block used in the casting process. These plugs, which are about 1.5 inches in diameter, will eventually rust out and leak if the coolant concentration is not sufficient to prevent rusting of the steel parts.
I think you have a hole developing in one or more of these plugs and it will become a real problem if you don't replace it and refill the cooling system with fresh coolant. With the park brake on hard and in park, safely slide under the truck with a flashlight and see what you can see and hopefully you can find the leak. It's possible that it's not the freeze plug but something else so trace the trail of coolant to the site of the leak and you'll have a better idea of what's wrong exactly. Coolant leakage is a real problem because if the coolant is low or suddenly blows out through whatever is failing can overheat your motor and ruin it.
Heres' a directory link to help you find a shop if you need one: