Just an update. There are two seals on the gear box. One on the side and one on the bottom. In order to get to the one on the bottom, the gearbox must be taken off the mower. I pulled the gear box from the mower. Which turned out to be fairly easy to do. In order to get the gearbox off I had to remove the center mower blade and release tension on the mower belt.
Then I ran into a problem. On the spindle on the gearbox is a belt pully which needs to be pulled off with a puller. I'm guessing the puller needed to be hydraulic. I considered buying one but I estimated the cost would be approx. $150 for a cheap one. I could rent one from the auto parts store for $65.
At this point I decided to take the gearbox to the dealership for a couple of reasons. 1) I was not sure if the seal was the problem or a symptom of some other problem (like shot bearings) 2) I figured the cost to be less if the seal is the only problem. 3) Finally, an expert could evaluate the problem.