I believe my trans has been leaking fluid from where the driveshaft slips in. What is that seal called? In case the seal is not the problem, what are some common reasons it might be leaking from the rear?

 

That seal is called an "extension housing seal".

If you're going with the shotgun approach, remove the extension housing from the transmission. Remove the seal and the bushing that's behind the seal and replace them both.

Before you put everything back, make sure you replace the big square section o-ring that seals the extension housing to the transmission case.

The fluid may also be coming from someplace else and blowing all over the back of the transmission.