What does this clue point to?

95 K2500 HD 5.7 TBI. Runs great.

I have been logging a slightly high BLM of 140-144. But noticed two things today that hopefully one of you folks will recognize as a clue.

1) At fully warmed up idle and WOT the BLM is normal. Cruising or accelerating it goes to 140's

2) When in Park it idles at 725 and the BLM is normal. As soon as I move the shifter to D the idle drops to 550 at the BLM goes to 144. But I mean instantly, as soon as I move the shifter.

Any thoughts?

1) BLM numbers indicate greater than stoich combustion under load.
2) Confirm BLM cell number when change happens. Sounds like two different cells.

http://www.gnttype.org/techarea/faq/BLMINT.html

There are some differences between this description and your vehicle, most notably MAP vs MAF, but the basic description is a good start.