I have a 06 2500 Quad Cab w/5.9L. The indicator for the transmission does not show P all of the time unless the truck rolls forward. Is this cable or the switch adjustable?

I did notice last night I put the shifter in P and the reverse lights were on until the truck rolled. Whats up with that?

With this happening is the truck actually in D? As I am cruising down the highway it does go into overdrive.