hmmm. Is it manual? It sounds like an axle shaft seal. There are only 3 shaft seals on the transmission, the input shaft, which is where the flywheel and clutch are where power comes in; and around the two axles, left and right.
For the input shaft seal, you have to drop the tranny.
It's probably one of the axle shaft seals. It's hard to tell which side is bad since the wind blows it around. The steps are:
1. drain gear oil.
2. take off wheel on each side you want to replace... one or both. Loosen wheel assembly, brakes, hub so axle can slide out from transmission.
3. with a pry bar against stub, where axle goes into transmission, pop axle out.
4. using a seal remover tool (buy at NAPA), its its like a hammer but flat (thin) and has a hook. Pry out the seal, its like a ring.
5. carefully insert new seal EVENLY, so it goes in straight. something round about the same width helps. tap in with a rubber mallet.
6. put everything back together. Grease around the axle where it will hit the seals--anytime you remove the axle. Same where shaft goes thru wheel hub and bearing. Fill up with gear oil.
Note, there is always confusion about LEFT and RIGHT axle shaft seals. The dealer usually has them mixed up. Maybe get both and bring your unused back.
Make sure the round pins on the axle ends that go into the transmission are in good shape; they shouldn't stick out too far, or not enough. You can get new ones at dealer.
The actual part is a few bucks, the real cost is labor.
Gear oil smells nasty! Is it leaking a lot, on the ground?
if its a minor leak you can get by a long time. You can check the level of the gear oil (on a manual) there is a large 21 or 22 mm bolt facing forward on the transmission near top. unscrew that and if you stick a finger in you should touch oil (do this went its not hot!). you can always put more gear oil in and check it. Get some extra long tubing.