How hard to fix rear differential leak?

I slid under the car to try and find my brake fluid leak, only to find that there's yet another leak . I think it's coming from the rear differential, but I'm not positive, there's fluid on all the lines and the exhaust as well. I don't think it's coming from the cover, it seems like it may be coming from the seal that the driveshaft goes in to. The only other possibility I see is that it may be coming from the unknown line (anyone recognize it?) in picture pix4.jpg

Anyways, has anyone replaced this seal before, is it difficult to do? Also, I have a leak where the driveshaft goes in on the transmission end, is this one difficult to fix as well? I tried to get as good a set of pictures as I could, if anyone can recognize where this problem is coming from I'd appreciate it.

thanks

http://www.chicagowebsearch.com/pix/pix1.jpg
http://www.chicagowebsearch.com/pix/pix2.jpg
http://www.chicagowebsearch.com/pix/pix3.jpg
http://www.chicagowebsearch.com/pix/pix4.jpg
http://www.chicagowebsearch.com/pix/pix5.jpg

Sorry, the line I mentioned is top center in pix5.jpg

Also, I have a question regarding my brake fluid leak, I noticed that there is a drop or two of fluid on the little rubber seal on the front of the (proportion valve?) thing under the master cylinder. Could this be a potential leak spot, or does that not hold back any fluid?