Hey:
If you filled it up a week ago and it's empty now, man it's got to be going somewhere. There is only two places it can be going.
It's either leaking around the seal on the housing cover, or out through the axle tubes which means that one or both of your axle seals are bad.
If this is the case you will have 90 weight grease all over your rear break pads and drums. If it emptied out check the inside of your rear wheels and tires. They should be covered in rearend oil.
Good Luck