From my experience, the rear axle seals ususally don't just blow out, the bearing starts going and wipes out the seal - either that, or it's the more rare case of the axle housing being bent or cracked. Check it out while you have it apart.
Yeah mine started to leak a little and when I had my axle apart to weld my rear last week I found my wheel bearings were shot, same thing happened on my 85 rear axle before i swapped it, bearings went and not soon after it started leaking