Hello,

In all likelihood, you will be overwriting and abandoning your 9365637 OS and replacing it with something else such as 12212156 that should work with your 1999 PCM. Certainly, make a copy of your 9365637 OS but you will soon say goodbye to it.

Try to segment swap a 5.3 liter LM7 2002 Silverado 12212156 and bring in the 4L80E from an Express van that also used 12212156. I have used LS Droid for my segment swaps.

For 4x2 conversion operation (I have no experience here), I should think that you will need to kill off the 4x4 DTC codes, if any. Although, I can't find any 4x4 information in 12212156 after a quick look. So, this might be relatively easy.

Rick