Most times wheel seals fail is from wheels being overheated ( braking on hills) or sitting for a long time and then used. Sometimes they just age out as well.
If you drive it this way you will have a real mess to clean up and yes you will have to replace the shoes. Right now if the shoes are not really oily I would use brake clean to clean them.
Like mentioned above check the vent on the top of the diff.they sometimes plug up and create pressure enough to cause a seal to go. The ony time I have seen 2 at once like that is from overheat.
