Andre, That's the pinion seal that's leaking. You need to remove the drive shaft, remove the nut holding the drive shaft flange on the pinion, carefully knock the flange off with a hammer and a large brass drift. It will not take much, so don't whack the hell out of it. Pop the seal out with a large flat screwdriver. Coat the outside of the new seal with RTV silicone and carefully tap it in with a small hammer. you will need to start it in one location and carefully tap it in going around the seal. Tap it in until it is flush with the housing. Coat the seal lip and the flange snout with gear oil so you do not tear the seal putting the flange back on. If it has a crush sleeve, you will want to get a new one and replace that before you put it back together. You will have to torque it back to spec. I am not familiar with the torque spec, but it is in the service manual. I know it takes a LOT of torque to crush the new crush sleeve.
You will also want to double check the pinion bearings for any play because it could be that the front bearing is bad and has caused the seal to go bad.
