Look in the owners manual to verify, but I believe the 2011 on up 2500hd trucks use a synthetic GL4 gear lube like 80/90. Last one I did took 2-3 quarts. I'm unsure what differences if any, there are between your transfer case and the 2011 thru 2019 trucks.
Your Owners Manual page 6-15
Transfer Case (Four-Wheel Drive) DEXRON®-VI Automatic Transmission Fluid.
Your Manual doesn’t give capacity but it’s 2 quarts.
I have used Allison Transynd oil and Mobile1 5w-30
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