Selecting 4 High or 4 LOW but no 4WD
 

2004 Chevy Silverado HD 6.0 Auto. 240 K miles. My 4Wheel is acting up and I had it off the ground to troubleshoot. I am not getting any indicator lights on the dash and the lights in the selector switch is indicating that 2H, 4H, and 4 Low are all being selected.
Initially when I switch from 2H to 4H the front wheels are engaged and turn for 3-5 seconds before stopping. In 4 Low the front wheels never engage. I tested and when 4H or 4 Low are selected the actuator on the front diff is working correctly.

Any experience in the forum with the Transfer case being engaged and then dropping out and not turning the front drive shaft after 3-5 seconds. When this happens I still get the indicator lights indicating the system is in 4 HI.
I am also able to get the system in N. I don't use 4wheel often but I am in Colorado and it needs to work. My first thought is to replace the selector assembly on the T-case. Looking forward to feedback Thanks.

My hunch would be the switch.

the switches so commonly fail that you can't find them in the junkyard. you know what never fails...a manual shift lever.

if you want to diagnose this....unplug the tcase connector and remove the tcase encoder motor from the tcase. move the tcase shaft manually with a wrench...if all the ranges work...the problem is either the switch, wiring or a weak encoder motor.

the tccm has full diagnostic capabilities...codes with be set to help with fault diagnosis....but you need a scan tool that can access chassis codes.