where do i check the fluid for a 4x4 transfer box on 2000 GMC Sierra 2500

my truck acts like it is holding back and not changing from 4 wheel to 2 wheel i was told to check the fluid to the transfer box where do i look for this at i have a gmc 4x4 serria series 2500 and when i put my truck in 2 wheel it does ok will it hurt it if i drive it in 2 wheel instead of all wheel drive??

Asked by for the 2000 GMC Sierra 2500
the fill hole will be on the transfer case'll see two pipe above each other.unthread the top one and just fill it with the proper fluid till it runs over and your full.the bottom plug is for draining.
thank you