Service 4wd light

Customer has a '00 Silverado 1500 - 5.3L - automatic.
 
Last year she came in because her Service 4wd light had been coming on.  We determined the push button switch to be faulty and replaced it.  After about 4 months, she said it came back on again.  She had one of her friends replace the actuator but that didn't help.  We got the switch warrantied but that didn't help either.  We drained the transfer case fluid and refilled it because she had never had it serviced; we had thought it was maybe the case itself but the replacement we got shifts manually the same as the one in the truck.  We've tried 2 aftermarket transfer case motors; those didn't help either.  We also tried running a new ground wire.
 
We're pulling our hair out over this truck.  One thing she mentioned was that previously when she felt the tires slipping, she'd hit the Auto 4wd button at 35-40 mph.  Any advice?

Welcome to the forum!!! Someone here should be able to help you, just hang in there.

hitting the auto 4wd button while driving shouldnt hurt anything unless you are mashing on the gas while hitting it.  if you let off the gas so the rear end isnt getting forward power, then hit the auto button then you shouldnt have any problems becuase the auto doesnt kick in until the tires start slipping.  maybe she was engaging it while she was into the throttle?

No idea...she just said the only time she used it was when she felt it slipping.  Any ideas?

Any transfercase codes? C0327 by chance? I don't know about the older ones like this one, but the newer ones are famous for C0327 Encoder Circuit Malfunction, which basically means it can't tell what gear the transfercase is in, either because the encoder sensor in the encoder motor is malfunctioning (you've covered that one already, changing the encoder motor) or in some cases the transfercase itself has parts worn out, causing the shift forks/shaft shaft on the side of the transfercase to become loose, which will throw the service 4wd light on.
 
I'm just throwing that one out there, may not be what's wrong in your case, but I see alot of this in the newer (03-05) trucks, amazingly with low mileage.

No codes at all; that's what so frustrating!
 
Is it possible that I'm just having bad luck with aftermarket encoder motors?  I hate the idea of buying a $700 one from GM and running the risk of it not fixing the problem.
 
Or is it possible that it really is something in the transfer case causing all of the aftermarket motors to burn out?  We thought it may have been so we got a used one but haven't installed it yet because when we shift it by hand it feels the same as the one that's in the truck.

The only thing I can think of is that it's the actuator but you said you replaced it.
Huh, no Idea

Clean the ground on the frame under the drivers front door, see if that helps. Get all the factory undecoat off of the frame and the ground wire there. If I remember correctly, the TCCM and the EBCM ground there, and if it gets corroded at all, they throw codes at you left, right and center. When you've got it cleaned off and bolted back on, either paint over it with a rust inhibiting paint or re-undercoat that spot.
 
Or they throw no codes at all. Sorry, forgot to add that in the first time.

I had a 2000 Silverado that wouldn't shift and the check 4x4 light stayed on. Mice had chewed some wires under the fuse box under the hood.