'92 4x4 with a Whine in the drivetrain

I have a '92 Toyota Pickup 4x4. It has developed a whine in all forward gears no matter what gear the transfer case is in. However, I do not have any whine in reverse. If I am in a higher gear and go to down shift. While the clutch is in and I go from say 3rd to second the whine gets louder until I let out on the clutch. I get no whine in neutral, so I do not think that it is the throw out bearing. Could it possibly be the bearing on the output shaft?