This is one of those jobs that will backfire on you real quick if you don't get it right... Any work involving the ring & pinion gears has to be very precise.
Removal of the flange will release the preload on your pinion bearings, so you'll have to make absolute sure you get it back together EXACTLY how it was so that the preload is restored back to what it was.
I suggest scribing a very clear mark on your pinion shaft, across the pinion nut, and finally onto the flange before you take it apart. After replacing the seal (and only the seal), put everything back together precisely how it was (realigning the marks when you tighten the nut) and you should be OK. Also be sure to clean the threads and use some Loctite on the nut too because once it's removed, it might not stay tight on it's own anymore (technically you're supposed to use a new nut anytime it's removed).
Now if you find your pinion bearings are bad (not uncommon if the seal is leaking), then I would suggest taking it in to a rearend shop (or replacing the axle with a junkyard unit). New bearings, as well as a new crush sleeve and re-setting up of the gears will be required in this case.
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