While I had my wheels off today getting new tires, I noticed there was a small amount of gear oil in the area of the hub surface where the wheel mounts against. The axle cap area and brakes were dry. I have noticed the smell of hot gear oil lately from that side. I assume I need to re-place the axle seal. Is there only one seal per side? Or an inner and outer seal? Only 28,800 miles on the truck.