Actually, they don't have a drain plug; just a fill plug. The easiest way to drain these types of diffs is to remove the lower-most bolt completely, and just loosen the rest of the bolts. Get a flat screwdriver between the cover and diff (where you removed the bolt) and pry the cover away from the diff just a bit. "Most" of the gear lube "should" drain predominatly from where you removed the bolt.
To seal up the cover after draining it, you have a couple of options. 1. You can buy a paper (cardboard) gasket from Jeep or a local auto parts store and use a little rtv sealant to hold it in place while you line up the cover and get the bolts started; 2. you can use rtv sealant only. Personally, I prefer the gasket and rtv, but either way will work. If you do decide to use only rtv, the best stuff out there is called "Honda Bond" and despite the name, is sold at your local GM dealership. It's grey in color and is not cheap (about 10-15 bucks a tube). GM uses this stuff in place of gaskets altogether on a lot of applications. Most dudes who work in the parts departments will have no idea what you're talking about, but if you talk to one of the shop techs, they'll know what Honda Bond is. If they don't just describe it as grey rtv-type stuff that costs a ***** load!!