I've always heard to spin your fan manually. Lack of resistance = bad fan clutch, so I'm told. However, my fan spun with resistance, so I dismissed it.
Truck had 75kish on it when I got it, and this problem started then. Given what I've heard/read/seen, I'd think if your truck is around that mileage, it is a strong possibility. When I replaced it, and the problem officially stopped, I ordered another to keep as a spare. Later on down the road, it'll be a life saver, I'm sure.