Typical AWD systems uses a center differential to provide power to the axles, they do not use a transfer case.
4WD systems uses a transfer case to send power. Transfer cases have two gear sets and require different lubrication (type and volume). They provide high and low 4WD power - there is no automatic high/low transfer case. The auto function for the Tahoe is to swtich between 2WD and 4WD (high) modes.
BTW, to me the below quote mainly stipulates when to check diff fluids. The manual should have a schedule when to change the diff fluid.
"When to Check and Change Lubricant
It is not necessary to regularly check front axle fluid
unless you suspect there is a leak or you hear an
unusual noise. A fluid loss could indicate a problem.
Have it inspected and repaired."
Just my 2 cents.