vibration , noise? 2000 cherokee

i just bought a 2000 model cherokee.. wondering if this is normal.. when im driving in 2wd i feel vibrations from the transfer case and a slight humming maybe a shaft spinning... it changes into 4wd with no problems.. so im wondering if its normal to hear and feel the spinning of a shaft in the transfer case when in 2wd??

Roham.