I had the same thing happen after having my rearend rebuilt. Id find one or two drops of gear oil on the inside of the rim quite often. There was no evidence of oil being "slung" so I assume that it only does it when the car is sitting. Both of the axles and the seals where replaced.
It hasn't done it for a while now so either it stopped leaking OR my differential has no oil in it...
It wasn't very much, probably two tablespoons leaked out over the course of several months.
My car gets tossed around pretty good once a month autocrossing so the gear oil DEFINITELY makes it to BOTH ends of the axles.
The weight of the oil should make no difference.
If they didn't replace the axle seal, that could be the problem. If they DID replace the seals, the axles could have grooves worn into them from the seals. Or as Slow4v said, they may have boogered up the seal reinstalling the axles. Crawl under there and wipe everything down and then see how much oil is leaking and how fast it's leaking.