Easy test,
start the car cold,
does the fan spin slowly or really fast (loud whirring noise )
then does it stay fast or slow down once it starts.
When it is working normally it will spin fast on start up like a plane prop then slow down. As the fluid settles in the clutch it is locked until it is spun , then it slows down and will only lock up again until heated to a set temp.
If it spins slow it may need fluid which is hard to find from toyota and harder to fill but costs $3.
If it is locked you cannot turn the fan by hand when the engine is cool and off. then you need a new clutch all 1990-2000 are the same only option is the towing package with a different temp fluid.
Switch to an electric fan like i did when mine locked up.