when I bought my 81 Sd I took it to the local (very good) lube place and had the transmission back flushed, when they went to drain and refill the differential they found that the trailer hitch
tubing that attached to the frame member the holds the differential was much too close to get a 14 mm
hex drive on ,fotinately they knew the routine of not taking the bottom plug out first. I went home and cut a large 1 inch bolt to about 3/4 inch in length then ground a 14mm hex on it leaving the bolt head to put a flat open end 1 inch wrench on I took it to them it fit easly and they could turn the large bolt hear with a big open end wrench.The owner of the lube shop traded me the refill for the home made tool.I generaly do
most of the needed work on my own cars but have found that the small extra cost of have gearboxes drained and refilled at a place with a rack and disposil more than makes up for the extra cost...
William Rogers.....