Here's a second guess for "Generic Wiring Harness".
On the Chevy's, we get something similar. The wiring harness is unique, but the main ECU, transmission computer, etc. is shared between Astro and Blazers. If you swap the Astro transfer case from AWD to a PT 4WD with a Low-range, the ECU freaks out when the revs don't match the output speed. It'll limit the revs and hold you in 1st or 2nd gear. However, if you add a wire to the correct pin on the ECU, and ground it when in 4LO, the "Blazer" part of the ECU knows what to do and the ECU is happy.
Thanks general!