[f5fstop]

Seal replace is about 2 hours, so figure it is an all-day drop off for the H3 at the dealer. They will replace the seal not the diff, unless it is something other than a bad seal. Seals that leak early in the components life, are usually damaged upon installation of the seal into the component during component assembly; or damaged when another component is installed (in this case, the shaft is installed inside the diff).
In this case, if the pinion shaft is slightly crooked when installed into the diff, it could slightly cut the ID of the seal, that over a short time will cause a leak.

(I guess I could add, it could be something else like a cracked diff in the shaft area, but that is extremely rare. It could also be porosity of the case, which also is very rare, so I would bet it is the seal, but not being there....)