I have seen a number of posts concerning the difficulty of changing the transfer case fluid on Discos. Most of these regading frustrations locating and removing the filler plug. Yes, there is in fact a filler plug it's located on the rear of the case just behind a cable wraped in foil. The plug does not have a hex rather a 1/2 drive drive indentation. It's very hard to access owing to it's proximity to a large drum on the back of the transfer case. I tried for an hour to get it out and gave up. I finally came up with a procedure that took less than five minutes!! I used one of those adapters you put an a 1/2 inch socket that has a hex on it. These are used for tight spaces. Put the hex adaptor right in the plug, you might have to bang on it a little if it's rusty. Once I got it stuck in there I put an 11/16 open ended wrench on the hex. Then I wacked the wrench with a hammer and it came loose after two whacks!!!

 

Great. And some owners back up on ramps, to give a little more working space (for chubby DIY), and allows you put a little more fluid in. I use a 1/2 inch ratchet and a short extension, have also used a "universal" joint style extension. I like your idea.

 

That "wire bundle" is the parking brake cable.
That drum is the parking brake.
The fill/check plug takes a 1/2" drive breaker bar or ratchet.
Same as the plugs on the differentials.