I had the same problem on my '47. It was recommended that I drain the rear end and clean the area where the torque tube enters the rear end. The first cleaning with a power washer and the second cleaning with brake cleaner. After everything is clean and dry, work some black RTV in the joint. Finish with a liberal bead of black RTV around the joint where the torque tube enters the rear end. This slowed my leak down; however it didn't stop it. I tried the process a second time with the same result.......
The torque tube is a pressed fit into the rear end and then riveted in place. I've also heard of people welding this joint to stop a leak.
My solution was to install an open drive shaft set-up on my 3100. That included a new differential (1955-1962), SM420 transmission, and a custom built drive shaft. The updated drive train solved my leaking torque tube!
Good luck,
