Hot Rod Lincoln explained it the best. The torque-tube is a different method of "driving" or propelling the vehicle. The enclosed drive shaft tube pushes against a bracket that's mounted to a crossmember in the truck frame at the rear of the transmission. The driving forces push against the vehicle near it's center, as Jerry wrote, rather than thru the springs. That's why the spring brackets have swivels on 3100s and Chevy cars. The enclosed front section of the drive shaft on 3/4-tons is a mystery. I think it was made this way so Chevy could use the same transmissions in 1/2-tons and 3/4-tons. Maybe not.
