Idk about a manual transmission but rear diff is easy as pie.

To completely get everything just remove the diff cover by loosening the bolts a little at a time, starting at the bottom and working your way up. If you start getting the bolts out and the cover isn't releasing you may have to pry on it a little. But don't pry too hard. You don't want to bend the cover. If you end up bending the cover a little like on a corner or something, that's okay, just put it against a flat surface and bang it back into shape. Let all the fluid to drain out. Then use some brake clean to get everything that's left. Then clean the surface of the axle where the cover bolts to it. Make sure it's very clean, and smooth. if there is any gasket material in the bolt holes, get all that out too. A set of picks helps greatly with this. Then take the cover and scrape off all the old gasket material (If there is any, depends on what kinda gasket was used). (I'm not sure if there is a magnet in there but if there is clean that off too).
Once everything is clean and all the mating surfaces are smooth use some RTV sealant to make a new gasket. Run a bead around the cover and around the bolt holes Like this:

Let the RTV set for however long it says too on the tube. Then put the cover back on and tighten the bolts. There is a torque spec for the bolts but just make them tight. Don't crank on them, but not hand tight. You get the idea right?
After the RTV is set (again, according to the tube), fill the diff with the recommended gear oil. If you have an LSD you will have to put friction modifier in there too.

Here is a pretty good write up I just found.

http://jeephorizons.com/tech/diffoil.html