the rear of the 4l60e case is open in a 4x4 application. The t-case is sealed at the front.
Often these adapters become warped and not flat where the gasket needs to seal. You can unbolt the adapter from t he back of the 4l60e trans. Flat sand that area on something large enough to bring the surface back to level.
A square cut O-ring seals up the t-case adapter to the 4l60e case
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.