There aren't usually any seals on a speedometer shaft, just some little grooves that sling the oil off the shaft. Your gear oil may actually be over full from water getting in the case somehow, the oil floats on top. You probably should change the oil.
The vent usually looks like a little mushroom and is on top of the case somewhere. Its usually a threaded fitting that kind of looks like the little thingy on top of a pressure cooker that regulates the steam pressure. It may have a hose barb and go up to the frame somewhere.
