That's the best way without dropping the diff. Use a drill pump or a hand pump and pump out the old fluid through the fill hole on the back/driver side of the diff. Once you get under there you'll see it. While that method does work, it doesn't get all the nasty out, only most of it. The right way to do it would be to drop the diff, pull the cover, do it the old school way. But that's kind of a big job, so just do the pump method and change it more often(shorter intervals).
BUT...if you've got a leaky diff cover, drop that puppy out of there.
'01 XLT disguised as a '03 Fx4 Level II.