When To Change

what is recommended mileage to change the front and rear differ.. fluid? and transfer case fluid? the truck in question is a 2004 gmc sierra extended cab 4x4 with 32,000
miles. the truck is not used for off roading. it has the 5.3 liter and the push button 4x4 system with auto 4x4. only thing i can find in owners manual is the transfer case change and that is at 50,000. thanks
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