To follow up on Stuart's response, there is no seal shown in the Shop Manual.

There is a rear bearing and a bearing support that might get worn. Stuart is correct about oil from the transmission moving back to lubricate the u-joint in its enclosure.

There are a few seals in the u-joint enclosure and a seal at the u-joint end of the driveshaft (in its enclosure). Are these the seals in question?

Maybe this is a trick question (just a joke)? If someone knows about a rear seal in a '50 1/2 ton 3-speed, maybe a picture could be posted from a Shop Manual?