What is my seal called?

Everybody has pretty much got you covered. I will add, when you put the yoke back on put some NON HARDENING thread sealant on the splines before you slide it on. I have seen them leak through the splines after they have been disturbed.

Here is a link for some service manuals:

http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=service+manuals

Open them and save them to your computer. I saved all of them.