where the mount is, is where the 4x4 adapter is. there is a rubber gasket, square style gasket. It's round but not like a O ring where it's ( o ) formed. I know I have about 3 of them sitting by my transmission. if it's NOT THAT, the. it's the other 5 bolt transfer case gasket.
to replace them is the same as removal of transfer case. start at back at drive shaft and work way forward to t casd. help is always good. safety is key when working under .
t case is full of fluid so will spill.
I just dug through my gmc typhoon receipt folder and found the item i ordered if it's the adapter gasket.
bought from GM transmission parts phone # 740-500-6099. website is
GM Transmission Parts & Rebuild Kits | Fastest Shipping Highest Quality Online
on 1034 - 700r4 transmission transfer case adapter gasket. I payed 5.87 each ended up buying 2. have a spare some where. that's a start if you want to go that direction.
best bet would be to ID where it's coming from exactly.