To produce the 1st gear ratio in L (Manual 1st gear), the reaction drum is held stationary by the brake band, this causes the reaction drum sun gear to also be held stationary.
When the planetary gear set input sun gear is rotated using torque supplied by the Torque Converter the planetary gear set carrier and attached tail shaft "walk around" the stationary reaction drum sun gear producing the 1st gear ratio. To produce the 2nd gear ratio, the 2nd clutch is applied and the reaction drum brake band remains applied. In 3rd gear and 4th gear (“Overdrive”) and Reverse the reaction drum brake band is released.
Therefore, if the truck will move backwards in Reverse, but L (Manual 1st gear) feels like you’re in Neutral and Winter Mode (3rd gear) produces forward motion the reaction drum brake band is either not applied or is worn out and is slipping. If the transmission has a lot of miles on it the reaction drum brake band is probably worn out.
The bad news is you have to disassemble the complete transmission to replace the brake band. However, sometimes you can get a few more miles out of the brake band by adjusting it as shown in the video:
