2003 Trailblazer BCM issue? U1064 code after v8 swap

[fakess]

The truck is originally a 2003 I6 Chevy Trailblazer. It's currently running with an LQ4 and a 4L80 using a Trailblazer EXT engine harness, and P59 PCM with appropriate changes in the tune with HPTuners for the engine and trans.
VATS was disabled, and starter wiring was modified from OEM to get it running and driving. It runs and drives great with no issues or CEL's, minus the constantly pending U1064 code that I haven't been able to solve. I'm hoping someone more knowledgeable than me with GM electronics might be able to shed some light...

-Everything works until the shifter is moved out of park. Lots of interior features are then lost (dash lights, battery gauge, interior lights, rear windows and sunroof, headlight high-beams). I also can't program the OEM keyless entry to get it working at all. All other dash gauges still work, A/C compressor activates and interior climate controls still work, cruise control still works, and all required exterior lights and flashers still work, even when pulled out of park and driving.
-I've checked power/ground/continuity through the body control module (BCM) and data communication system multiple times over, since it is after-all, a swap...
-U1064 is the only code pending, no other CEL's or warning lights
-The PCM vin# currently does not match the vehicle. I don't know if this matters for communication with the original BCM in the vehicle, or not.
-I tried installing a used BCM from a correct model year (for the PCM vin) trailblazer EXT with a factory v8, with hopes that a BCM that was at least looking for the correct PCM and some of the same RPO's would get me somewhere. It didn't, and I had all the same symptoms as well as an additional flashing security light, and airbag light lit on the dash. I then put the original BCM back in since I daily drive the truck, and can't stand warning lights on the dash.
::Side note:: While flashing the PCM, the "use high speed" feature never works. I don't know if that is related to any of this BCM stuff, or not.

Does anyone have any suggestions as far as which aspect of this I should be looking at changing next?
-VIN change in the PCM?
-Do the RPO's just need to be correctly programmed in the BCM for this to work right? (and is that even something a dealer would do?)


Any insight would be appreciated, thanks for looking

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it seems the bcm is not liking ecu and tecu. Which bcm are you using now? Can you put in a BCM from a h3 that came stock with the 5.3?

[fakess]

Keep in mind- this is all GenIII stuff, not a Trailblazer SS (there is no separate TCU). It currently has the original 2003 I6 BCM that came with the truck

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then a h3 5.3 with a manual trans would be the bcm I'd try.

[fakess]

If a Trailblazer EXT w/ factory 5.3 BCM didn't work, why would a Hummer H3 BCM work?

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The GM early mid 2000s used the ip cluster as a talking comm in between the bcm and pcm. The bcm configs the ip, pcm does not. You might just need to have the bcm recalibrated to match a new ip. Unless the ip can be reset for the bcm-ip config?