01 Sierra: 4x4 no Longer Works

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2001 GMC Sierra SLE 4x4 4.8. It has the push button 4x4 setup with Auto 4wd.

Owned the truck almost a year. 125,000 miles. 4x4 worked great last winter. Not working this year.

Couple days ago during snowstorm I selected AUTO 4wd and was about to start my journey. Truck was slippin and sliding and clearly not in 4wd. Just as I started to recognize the condition, the Service 4wd System light came on. I pulled over to a side road. Shut the truck off and restarted. Service 4wd light went away and this time I selected 4 HI. Note: The service 4wd light would never return again.

Still not working in 4hi even though the light is illuminated on 4hi. Ok great. This time I try 4 LO. Light is illuminated on 4 LO and the truck is clearly in low range because the truck is much slower but torquier. BUT, still no traction, front wheels not engaging.

I can hear the transfer case doing its motor thing and engaging between all the 2wd and 4wd options. I removed the front diff actuator and that is functioning fine (retracting and moving). I decided to put a new one in (Dorman) after removing the original because the plastic threads got kinda messed up.

Still no 4wd. I jack up the front of the truck. In 2hi each front wheel is independent from one another. I engage any of the 4wd options and yup, the diff is definitely engaging because now the front wheels react to one another when the truck is jacked up.

I had a friend carefully drive the truck while I observed the front drive shaft. It spins although I also notice it spins in 2wd so not sure if thats a good test. I read somewhere that these NP246 T cases will provide some output to the front shaft even in 2wd due to the clutch pack inside.

All 4 wheels on the ground if it’s in 2wd I can spin the front driveshaft. In the 4hi and lo modes it definitely locks in.

Ok on to the transfer case fluid. Drained it out. Black and BURNT like real burnt stunk up my garage. I replaced with new Auto Trak II and tried to cycle thru the modes a few times and drive. Still no 4x4.

Is it safe to say my T case is screwed? Is it possible the T case motor although it seems to function that it may be broken? I would think the selector lights would be blinking and not going into the different modes if that were the case?

Think I bought a lemon
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