Although seemingly crazy, raising the drivers side of the car .540 metres with a couple of floor jacks, then adding some heavy duty 6 ton jack stands for safety.....the level plug on the driver side of the transfer case can be removed and 75 - 140 synthetic gear oil can be added quite easilly.....with the car on an angle the oil level reaches across the transfer case and fills to the correct level.
The other way to fill is use a pressurised vessel to hold the appropriate amount ( 600 ml I believe ) of gear oil and once this has been pushed in, quickly reinstal the level plug.
The cooling duct for the transfer case may be missing in action...and I believe should be found or purchased to help the transfer case stay cool on the highway at speed....or the small amount of lube in the t.c. will evaporate....or something similar and t.c. failure will undoubtebly eventually occur..... .so however this should fasten in place with one bolt just behind the engine oil pan....and the left side slips into an indent ...........
There is no drain plug in this assembly period, in my manual it states to remove the driveshaft and remove the oil seal at the rear of the t.c. to fully drain the oil.....I think after my experiences that simply topping up the existing fluid , esp if you let any existing oil drain while the car is horizontal from the side plug, should do ....
Personally I went one step further as others have and carefully drilled and tapped a 1/4 inch thread into the lowest area of the t.c. , this really got the last of the old oil out for sure...I then added several ounces of fresh oil to remove any swarf ....installed a short coarse thread bolt with sealer , and pumped in the 600 ml of synthetic gear oil....