Need to remove the belt from all the pulleys on the front of the engine.
Then spin all the pulleys by hand 1 at a time, They should be smooth and easy to spend and quite. If not then replace the one that's not.
Open the glove box and let the squirrel out!!
Most common culprit is a slipping belt.
Look to the alt. belt. It may squeal under load.
did you check the water pump, alternator and anything else that might have a pulley?
It could be the transmission, it could be some parts under the vehicle that rub only when travelling forward.
Under the vehicle, behind the driver.
under the pickup in the center of the engine block
under the dash on the passenger side.
Under dash, left side.
under drivers side seat. under the truck.
It is under the dash where the passagers feet go.
Usually passenger side, under the dashboard.