If the acceleration in Drive 1st gear is the same as in L (Manual 1st gear) and the engine revs up but truck does not move with Winter Mode selected it is most likely a burned up 3rd clutch. The repair is to remove and rebuild the transmission.It does not accelerate from stationary in winter mode.
it does accelerate in 1st and low from stationary.
Winter Mode is 3rd gear. To produce the 3rd gear ratio the 2nd clutch is applied causing the ring gear attached to the 2nd clutch drum to rotate the planetary gear set long pinion gears at the same speed as the 3rd clutch drum. The 3rd clutch is also applied and drives the planetary gear set short pinion gears. The long and short pinion gears are always mated together. When the long and short pinion gears are both being driven at the same speed, the planetary gear set is locked and the planetary carrier and attached tail shaft rotate at the same speed as the torque converter, this produces a 1:1 ratio which is 3rd gear. If either the 2nd or 3rd clutch slips the planetary gear set is no longer locked and engine torque "leaks" out of the slipping clutch. The engine revs up but the truck does not move. I am reasonably sure that replacing the valve body will not fix a slipping clutch.I wouldn't be so dead-set that the transmission needs rebuilt. If you can find a donor valve body on ebay / car-part.com etc, I'd swap that first. It could be a control logic issue rather than a clutch material problem. Replacing the valve body can all be done under-neath the vehicle without removal of the transmission. An evening job.