The fluid in the transfer case is to keep the gears cool and lubricated. of corse it doesnt take as much abuse as an offroad vehicle but wear still occurs. not only do you change transfer case or any other fluids, for lubrication purposes. but you also change the fluid to rid the component of the metal shavings that occur inside of mechanical devices. those metal shavings are one of the causes for premature failure. Do you have to change it at 50K? I would. just to stay on top of my maintenance if nothing else. some people dont change it in 100k. but the longer the same fluid stays inside the m ore wear is caused.