transmission seal

It appears the left side stub axle seal is leaking on my 5-speed. How does one replace the seal? Also, what would normally cause this seal to leak, I have never had one go bad and I have had way too many Xs.

When I bought my '86 Bertone a year ago I noticed a wet area on the drivers side of the final drive case. I asked the previous owner about it and he said he thought it was coming from a weepy drain plug. I thought it might be the axle seal above but the two are so close together it could be...