Need help/advice with P/N and unstable Idle...

I am in the middle of trying to tune this P59 DBW FlexFuel ECM for my rock crawler. I don't have a dyno so I'm tuning it by driving it trying to hit all, or most, of the conditions. My problem is, everytimne I come to a stop it will die. It will start right back up and idle good, as soon as you come to the next stop, or blip the throttle while still it will die.
I have the idle set to about 785. I don't have any signal going to the ECM that tells it it's in P/N or in gear. The trans is a 400T.
I'm thinking I need to rig something up to let the ECU know it's either in P/N or in gear to fix this but I'm not sure.
If I need this, how does the P/N function work with this ECM? I think I understand that it uses one wire for this function, and uses a ground signal, but is the wire grounded in P/N or in gear?

Can some of you knowledgeable folks on here help a guy out? Thanks so much. Can post the tune if I need too this evening.

I believe there is an option for PRNDL ( trans section?), None or P/N switch for that PCM...use a micro switch to switch ground ( I believe P is ground)

Thanks orange ss. That's what i seem to remember reading somewhere but I am not sure. I was hoping someone would have first hand knowledge on how to rig a switch up like that.

Okay, so I figured out the P/N issue. With my P59 ECU I set "System>General>PRNDL Equipped" setting to "P/N Only". I had to do a "Write Entire" to get this change to take. Does this sound correct? It's the only way I could get it to happen.
I then loaded the scanner and brought up the "PRNDL Status" in the scanner. I reinstalled the white wire back into pin 32 of the blue connector and then watched the data for that parameter. With the white wire GROUNDED this is P/N, with the wire open, it shows "D4", so I'm assuming D4 means "in gear"..........

That's what I'm going with. Hope this is correct and this information helps someone else out in my, or similar situation