Why would rear differential overflow after 7 yrs/111k miles

The next time you drive it, stop and check if the differential case is getting hot.

Rear differentials are impossible to overfill. The fill hole is horizontal and the prodedure asks to fill up flush with the hole's bottom side. Mechanics usually fill until the oil starts to overflow. Put anymore in and it will immediately leak to the ground. Did they open it to check the oil level?

Should it not be hot? Would that be indicative of an internal problem?

And how hot would be too hot? Hot to the touch?

Thanks.

If it's hot it may have boiled the oil which would force some out the vent hole. It could be hot due to a failed bearing, unequal tire diameters or dragging brakes.

Not submerged, no. I drove through some puiddles the day or two before, but nothing more than a few inches deep.

Would gear oil float on water like light weight oils? Or vice versa?

not so much the oil floating, just that if you made a "water crossing", especially with the differential at operating temperature, deep enough to cover the vent, water could be sucked into the diff. if enough was sucked in to fill it, oil would be forced out eventually. that is the way to "overfill" a diff, ie, through the vent tube. not sure about subaru, but many diff. vents have a check valve to prevent this.

tim