I noticed that the axle seals at the differential are leaking diff fluid.

So, I drained the fluid and replaced it only to have it leak even more out.

I did overfill it since I only had the driver side of the car on a ramp and I kept going until the fluid ran out the fill hole.

Having said that, does the diff have a vent or did it likely just come out the existing leak?

Now, to the more interesting prospect:

Is there a higher geared differential that I could install for better fuel efficiency on the highway?

I drive about 5K miles a month so I don't care a bit about off the line acceleration.

A boost in MPG would be very welcome.

Since I will likely have to pull it anyway to replace the seals I might as well upgrade if I can.