Rear differential leaking

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Well I went to change the transmission and differential fluid on my newly purchased 68 C30... wouldn't be surprised if that transmission oil was original... but as for the differential, there wasn't much changing it because it was almost empty. I filled it up yesterday, and this morning I see why. It drips like a faucet (1 drop every 15 seconds or so) from just behind the u-joint on the front of the differential. Is this something I can fix myself? Anybody know where an exploded diagram is so I can picture this seal? Thanks!

I'm pointing to the leak in the diff pic. Also had to post one after I cleaned the lichens off the paint. Still need a bumper. And not sure where the seat in the truck came from, but who doesn't love a gold-sparkle steering wheel?

*edit: I guess this is called a pinion seal? I see where I can order the part, but should I tackle this without air tools?