Getting the correct fluid level / amount in your diffs is not an exact science and it is not absolutely necessary to have the vehicle level, within reason, to add fluid to the differentials. As long as you don't add fluid from the vent, you will not overfill it. You are more likely to under fill it, but you would have to be on a sloped surface with the front lifted even higher to run that risk on a normal street vehicle.
If you want to just put jack stands under just the rear axle while you service it, that is fine.
It is much more important to ensure that you get any debris or dirt out, if any get in when removing the cover. The is a little more difficult with the rear lifted, or if the suspension is at ride height, but is still completely do-able.
If you have a limited slip or posi unit in the diff, it is much more important to ensure that you have the correct amount of friction modifier for the unit that you are running.
Heck, as long as your cover is in good shape, even the gasket is optional.
