04' z-71 4x4 problems.. service light comes on

hey good day to ya'll
 
my dad is having some problems with his 4x4 on his 04' z-71..
 
the light comes on that says service 4 wheel drive.
 
the manual says turn the key off and back on to clear, if it doesnt clear dont drive it and service asap.
 
he shecked the fluid in the case once, it was low. filled it up and now it's low again.. it was about a month since he filled it up the first time.
 
any clues as to what we can do to fix it? or maybe where the fluid is going..?
 
he didnt say he could see any dripping or anything..
 
anything is better than nothing... and thanks in advance! ! !
 
(i couldnt find a z-71 area in the forums)
 
austin