Early trucks ( 2001-2003) came from GM with a vent cap in the front diff that may not be compatible with synthetic fluid. In fact GM issued a TSB outlining that the vent cap should be replaced if using synthetic fluid. This is only needed if you do not already have the white cap.
Its also common for these to work themselves loose and cause a leak that looks like it is coming from the left front axle seal. Easy install, threaded and comes with proper o-ring seal.