If you still have your belt fan installed on your truck (can't remember if you have officially switched to electric fans yet), that is the water pump that the fan is attached too!! Just go under the truck and locate that fan, then position yourself in such a way to get a good view of the base of the water pump where the shaft goes into it and at the bottom of the base there should be a couple of small holes, if there is moisture there than that is probably your problem!! Best to try to wipe it off as best as you can and drive it for a day and look again, if it's wet then it's time for a new water pump!! Change out the thermostat while you are at it and examine the hoses real good, cheap insurance!!