You can instal a coolant flushing system. You put a tee into the heater hose coming from the water pump to the heater core. They can be purchased at most auto part stores. (Prestone IIRC)

or
You can remove the radiator and flush it but it can be done in the truck too, but not as effective. Push water though the bottom outlet, watch the crud come out through the top opening. The higher the water pressure the better.

 

awesome, thanks