this is zachs buddie not him just so u know. I have a 2005 gmc duramax and i just replaced the prindle switch and its still acting up. not wanting to go in R and or D just every once in a while. I brought it to miller transmission where i live and they said its some harness and to get to the end of it i have to take apart the bottom end of the transmission. I am trying to find the harness but i can not remember the name I wanna say tis the internal wiring harness. i need help guys

 

Could be your tcm (transmition Control Modual)

 

Neutral Safety Back Up switch or NSBU switch, that more than likely the problem, its quite common on the 5-speed allison