HCE--You're right, it locks out the overdrive, not the 4x4. I misspoke, but I meant what you said....
Ok guys, I got a chance to work on the GV in the truck tonight. I removed the instrument bezel and was able to remove the dash switch and do some testing.
So behind the Autodrive light there's a yellow & green wire. Behind the Overdrive light, there's a yellow & blue wire. Behind the on/off switch there's a black, red & white wire. So the first set of wires I was going to test was the yellow & green autodrive wires, which is the red light was wasn't coming on. I had a hell of a time removing the butt connector off the spade because it fit so tight (which tells me it's not a loose connection issue). But once I moved it a little so I could get my multimeter test probe on the spade terminal, I flipped the switch on the gear shift and the light came on. So it seems that even though the connections were super tight, just moving the connectors made it make contact again? Who knows, but the light came on. I haven't driven it yet to see if it will shift, but the only reason it wasn't shifting is because the red light wasn't coming on in the first place. So even though I got it to work, I went ahead and did some voltage testing on each of the spade terminals with the gear shift switch in both the on and off position. Here are the results.....
DASH SWITCH AUTODRIVE & OVERDRIVE WIRING
--Autodrive Yellow On: 13.52v
--Autodrive Yellow Off: 13.68v
--Autodrive Green On: 0.15v
--Autodrive Green Off: 13.68v
--Overdrive Yellow On: 13.47v
--Overdrive Yellow Off: 13.64v
--Overdrive Blue On: 13.50v
--Overdrive Blue Off: 13.68v
DASH SWITCH ON/OFF WIRING
--On/Off Red On: 13.55v
--On/Off Red Off: 13.64v
--On/Off Black On: 0v
--On/Off Black Off: 0v
--On/Off White On: 13.64v
--On/Off White Off: 13.70v