1992 Toyota Pickup will not go into gear

I was not driving the truck, but my brother was. He said that he was going up hill and had to downshift from 5th to 4th gear. When he let out on clutch the truck started slowing down and the engine reved up like it was not in gear. We got the truck home and I checked it out. You can push in the clutch and put into any gear and let out and there is nothing. You can also go into any gear without pushing in the clutch. You can do all this while the truck is running. There is no sounds of grinding or any type of resistance. The clutch pedal pushes in and out fine. Fluid level is fine.