Air in your coolant system.
Try parking you truck on a slight incline, or jack up the front, open the rad cap and idle the truck till it is warm and the thermostat opens. Rev it to about 3g and hold for a min, then let it idle for another 2 mins, repeat that 4-5 times. Then make sure you top up the coolant res. Keep an eye on the level of the res for the next week.