You don't have to take the whole gearbox off the chassis to service it but it's always better to do it that way. You could find something else that needs fixed.
But if you need to just take out the diff in the rear you can just take off the back part of the diff case. On the bottom just take the back 2 screws out and undo the sway bar and the screws on the back of the diff case and the back brace, you should be able to pull the diff out. I know you can dobit cause I've seen a guy do it. The front should be the same but do the front of it. The center, the is no easy way. You have to just take the center part out unless you have the quick mount for it.
To me it's just easier to just take the whole assembly off to get to it.