WHEN I AM IN DRIVE MY TRANIE SCREAMS AND MAKES A REALLY LOUD GRINDING SOUND! WHAT COULD IT BE?

Automatic ? Worn Out Main Bearing in the Transmission, if it is 4x4, check Transfer Case too, best if to put your truck on block, no wheel touching the floor, put it in gear and check underneath where the noise is coming from ! Could need to Replace the Transmission or Replace the bearing inside, check if it has full oil too.. !