Rear differential leaks

I have a '96 2 wheel drive suburban that runs low on rear differential gear oil about every 12000 miles. I can tell its low because I start getting the drivetrain bump that I've read about in so many posts.
Once I top the level off the bumping stops. What is causing the loss of level? There are no leaks visible on the differential or on my driveway.

I had a similar problem but with a Chevy van. Differential gear oil was leaking into my brake drum via the axle seal. I would remove the rear drums and inspect if you do not have any external signs.