It is not likely that anything is fooked unless of course you have reason to suspect something. Any strange vibs? noises, like growling/whining? If not then don't mess with it.

the pinion nut is 1 1/16", I would get a new one while your at it.

Stock 9" don't have a drain plug so you have two ways of doing it as far as I can figure.

Option 1: remove one side brakes and pull out the axle shaft. Then jack up the vehicle so the end with the axle shaft removed is now significantly lower. This will drain the majority of the fluid out although a bit will be caught in the lower diff hump. Reinstall axle shaft and brake on one side and re-fill.

Option 2: Remove both side brakes and pull both shafts. Then drop the d-shaft and pull the third member. Then pump the fluid out of sop it up with sponge/rag/whatever. Then reinstall third member, d-shaft, axle shafts, brakes and re-fill.

I would go with option 1: unless of course you have extra work to do anyway. Like changing brake pads, fixing a leaking third member gasket, or installing that solid shim for the pinion preload I was talking about. So to answer your question, No, it is not true you HAVE to pull the shafts and the third.