You might be able to remove the pinion nut and just replace the seal. However, you run the risk of not having the correct preload on the pinion bearings when you reassemble because the crush sleeve is only supposed to be used once. Some shops reuse the crush sleeve and just loctite the pinion nut.
When mine started leaking, the diff shop disassembled the entire diff and put in a new crush sleeve to ensure the proper preload.