maybe the range selector switch on the side of the transmission behind the shifter cable arm is having issues. check the plug and wiring there or maybe the switch is bad.

 

Clean the ignition switch and connector contacts, and clean the TR switch and connector contacts:

 

141,000 miles by the way 08 Alpha

 

TMI...Take the free advice to correct your issue, or throw part at it.

 

I don't know what got under your saddle blanket HUMMERZ, I was just responding to a question from Doc Olds with the mileage...And yes, I will take the advice...LIKE I ALWAYS DO...THANKFULLY!!
I am familiar with that switch, I've had it on my bench before.

 

if you have suspected it in the past then maybe check it once more or the connectors too it.

 

Would a door sensor or speed sensor make sense here?

 

UPDATE: So I replaced the BCM...no joy. Then I replaced the shift position switch on the side of the transmission...again no joy. Replaced the Transmission control module...again no joy.
Nothing is working. From the wiring diagram, it looks like the shift position switch wires go directly to the TCM. I'm out of ideas, and nearly out of money and patients. Where do I go from here?? Any thoughts

 

then you really need to go through your harness, powers and grounds for damages or corrosion

 

Throwing parts at it=WASTE OF MONEY!