Originally Posted by Gilly

Brake light switch. If it shifts out using the override and then it operates normally, it's probably the brake light switch. See if the brake lights work. Even if they do work it could still be the switch, because there are 2 sets of contacts. One will let the brake lights work (the normally open contacts), if the normally closed set go out the brake lights will work, but the shifter and other components of the truck need to see both sets working.