Chevrolet Repair: No 4 wheel drive, differential action, refference

Question
1992 GMC 350 auto 4x4 pickup

When I shift into 4 wheel drive I do not get the "in 4 wheel drive light on" and it does not go into 4 wheel drive. No noises of other problems heard or seen. What should I look for to fix the problem. When I move the 4 wheel shifter, it feels like it is engaging, but no light on and only two wheel operation.  Thanks

Answer
Hello Larry,
Normally, that would take me to the front axle actuator.
You feel engagement, because the transfer case shifts.
But it should close a switch, which in turn sends power to the thermal actuator, which couples the front axles to the front differential internally.

www.4x4posi-lok.com/PDFs/troubleshootguide.pdf

offroadltd.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=ORL&Product_Code=PSL600

www.4x4posi-lok.com/

These sites show how it works, and of course they are trying to sell you their replacement system.
But I am still running the orriginal equipment type in my fleet. occasionally need to replace an actuator, but there are some of my trucks that have never needed it.

There is one refference in the troubleshooting page that talks about a constantly driven axle, and the free wheeling right axle...they both free wheel until the actuator locks the right one to its side gear, and then normal differential action takes place.
But all the proceedures to test it are correct.
Good luck,
Van