When was the last time you replaced the water pump?
5yrs ago. That's 1 of the reasons I have the coolant flushed once a year. The previous water pump was DESTROYED. ALL of the blades were completely rusted away, and the coolant was full of junk. I flushed the system 2 times then, AND replaced both heater cores AND the radiator at the same time (might have been 6yrs..not exactly sure, lol). Regardless, it was right after I bought it, and I knew if the water pump was that bad, then everything else was more than likely clogged. I REALLY don't want to think the water pump needs replaced again, but I'm getting to the point that almost anything is possible with this truck. Luckily, changing it isn't THAT bad.