What year is your truck? What tranny? Any leaks? Any fluid on the ground? Any Fluid under the truck on the components? Are you loosing transmission fluid (noticable from the dipstick under the hood) or transfer case fluid (noticable by removing a fill hole plug under the truck)? How many miles?
Yeah the most common transfercase leak is the pump rub as mentioned above. Merchant automotive ( GMDieseltach.com ) sells a kit to cure the pump rub and also sells the parts to fix a allready rubbed case!