Does it have rear abs?
Try and get the part # XXXXX the computer, its by glove box in r/s kick panel.
That is a 340E trans, There is a speed sensor in the speedometer head that affects shift points and quality.
Yes, The truck will not shift properly without it.
I can't really tell you what damage it'll do, or how it manifests. Without it, the computer does not know the vehicles speed. I would go with yes.
Yes, The computer will not know the speed of the truck, causing the torque convertor to not lock up, the car will not go into closed loop. The trans will overheat, from not going into closed loop.