The only tube coming off the transfer case would be for a vent line, usually there is just a little vented cap is installed in the top of the rear output shaft housing. Some people replace the vented cap with a fitting so they can connect a hose to it, and to run it to some higher place to keep water out when fording. The tube you are talking about maybe the speedometer cable, which also comes off the side of the transfer cases rear output shaft housing. It would help if you posted a picture of what you are talking about.
Thanks for the reply yes there is a vent cap on top of the transfer I would assume the tube coming off the rear section of the transfer is for a speedo cable. it just appears to be excessive looking for a cable port.