When you change the gear oil in the axles, loosen but don't remove the top bolt in the cover. This will keep the cover from falling, and gear oil from splashing all over you!
Check for metal in the oil on the way out (I like to drain over an old magnet I keep just for that reason) and give the inner case a quick blast with some good carburettor cleaner or similar to get rid of any accumulated crud.
Install new cover gasket and cover, and refill. You will be putting in almost a quart per axle, IIRC, and you fill until it just starts to weep out of the fill hole. I found a "suction gun" at my local hardware store for about $10, (think 1-pint syringe) that makes filling the axles, tcase, and manual gearbox miles easier. Something to look for - it's money well spent!
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