2000 blazer 4x4 automatic

when i put her in 4x4 the transfer case is leaking ATF every where its spraying it all over bottom of fuel tank and all the way to the rear diff.
Only happens when 4x4 is on, where should i look or has anyone had this issues?

Thanks

 

check output shaft seal to front driveshaft? I dunno

 

have someone else put it in 4wd while you look underneath