The radiator fans use a viscous clutch setup. Fan spins all the time just from rotational momentum of the water pump and air being forced into the front of truck at speed when the clutch is disengaged. It spins but spins freely of the engine and at a different rate if that makes sense. When hot the clutch engages using a temp sensitive spring and mostly locks the fan with the WP shaft and turns with the speed of the WP/engine in an effort to draw in more air across the radiator and cool things down faster during slow speed operation. I think it also locks up on startup of the truck. Mine does not work so I have never heard the 'roar' of the fan at startup other members speak of since owning the truck. I think there were direct drive and clutch drive fan options offered, but I am not sure which trucks/years got what most have clutches. The clutches go bad as they are engaged using the spring and silicone fluid in the clutch and the fluid can leak out over time or harden up causing them to no longer engage properly. There are threads on replacing the fluid on here. The fluid is actually considered a wear item. Mine is currently bad and I am replacing it while doing some other cooling system upgrades. Common issue on 60 series. Possible yours is just locked up for some reason. See if, with the engine off, you can move the fan freely or if it is hard to move. Rock auto has the aisin clutch (same as Toyota OEM) for like $70 I think. P/N FCT017. HTH.
