Hi my name is Hal.
does this truck have an Allison transmission?
manual does not say vin # XXXXX
all I can tell you is it's a 3 speed auto with overdrive and has a push putton on the end of shift for towing also isXXXXXand I think is a 8.1 liter engine. also the P-R-N O- 3 -2 -1 on the dash is not showing which gear it is in except 3. will not go into reverse from park without putting in a forward gear first, it will auto shift in 3rd through 1st, 2nd to 3rd, when I put it in 1st then slide up to 2nd, 3rd to overdirve the 3rd is lit even though it should be in overdrive as the next is neutral.
If it has an 8.1 v8, it most likely has an Allison transmission. I can check for sure with the vin. Thank you for providing it.
I believe your concern is the park neutral position switch. ( PNP) it mounts to the side of the transmission, and is a very common failure . You have to be car full when changing it though. where the shifter lever bolts on it will deform the shaft slightly and create a burr on the shaft. Just use a file to smooth the shaft before installing the new switch, or you may damage the new switch. It needs to slide on very easy and smooth.
Just checked the transmission is an Allison. also found that you have two recalls open. one for the crankshaft sensor and the other for the tailgate cables. any GM dealership will repair these items for you at no charge.
If you need more information Please Ask.