I actually just finished dealing with a leak at this exact same spot. But, in my case, i took the housing apart and the O-ring is what was completely in pieces. So I went to local autozone and got me a pack of o-rings of different sizes, and replaced it and viola!

In your case, if your leak is in this area, your coolant would be leaking on top of the tranny/transfer case. Its a bit of an annoying area to work on. The way I went about it is i loosened up the pipe that goes from the pcv valve to right side of the engine into a 3-way T type adapter. You can remove it to make things much easier but on my car car, this hose is very very stubborn and it felt like its welded on to the 3-way T, so i decided to leave it alone and just do it the hard way. That part is held on with 2 10mmx5/8" or so bolts to the engine block, and then the other side of it has the 2 same type bolts holding down the top radiator pipe that goes across the engine, and it also has the 4 wire coolant sensor on top, mine had only one sensor, others i hear have 2 different ones on that same piece,as CCA4's links points out.

It really isin't difficult, its just annoying if you wanna do it without removing anything else, but you wanna venture on removing that pcv pipe i mentioned, this will be much much easier, nothing else in my opinion is needed to remove this piece.

Oh wait, yeah......to more easily reach the bottom of the 2 flange to engine block bolts, i also removed the exhaust heat shield that's attached to the engine.

Once you remove or even loosen the 2 bolts, coolant will start pouring out, not much though, maybe about 20-40oz of it probably, obviously when its done when its cold, when the car has been off for a while. Once you start the engine and only run it for even just 5 minutes, you have pretty much charged the whole coolant system and more coolant will gush out. You can PM me if you have any questions, i have done this procedure on 2 different cars now........................