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So it is the 4.2 and when you turn the switch from 2wd to any of the other options, the indicator light just blinks then goes back to 2wd. I can hear a solonoid or something click under the truck.
My scan tool has a drivetrain status pid that does not change from 2wd when I move the switch.
I have good 12v on the vehicle side of the 4wd actuator. I was getting varying resistance readings on the signal wire in that harness when changing the switch settings, so does that indicate that the switch itself is working?
I saw a tsb that has you look at a 36pin harness for bad pin connections but they look ok.
No dtcs.
What's my next step, take a look at the transfer case motor?
You can run it by a transmission shop and most of them have a scan tool that can communicate with the transfer case shift motor which is the indicator of what you're describing as I've had the same issue on my 2004 Chevy Tahoe.
I was able to diagnose mine because I would occasionally get a service four wheel drive in the driver indicator display in the dash and when I hooked my scan tool up I was able to manipulate the shift motor and see that it wouldn't engage in two different modes.
You can remove the switch from the dash and then plug it back in and do the same at the shift motor on the side of the transfer case. I have a NP 246 transfer case and the motor is on the driver side with a black harness plug coming from it that is plugged up around the top side.
As I stated above though someone with the advanced scanner tool can easily actuate that motor and tell you what's going on most likely. I use the modis edge by snap-on my self and that's how I was able to diagnose it.
IF it has bidirectional control you should be able to activate it while it's in the vehicle as well.
Sometimes it could be the switch that controls it as well but normally it turns out to be the motor from what I've been told. Does the vehicle have a DIC where it would display a message? I knew mine was amiss when the dic showed a message of "service 4wd" . The bidirectional will allow you to see what position it's in if your tool allows that which should make troubleshooting easier.
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