There is a seal in the torque tube that is suppose the stop the migration of transmission oil into the enclosed shaft and into the rear differential.
When this seal fails many replace with a Okie bushing/seal like this one:

https://www.classicparts.com/1951-54-Drive-Shaft-Bushing-Seal/productinfo/92-012/

If you transmission seems to be low on oil a lot and your rear end is over filled then you need this new seal.

Steve


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration