The Stovebolt Site Forums The Shop Area Driveline Torque Tube and Differential Leaks

The leak at the collar means you need a new cork gasket under the collar. Or possibly you just need to snug up the collar.
As far as a leak at the differential, it could be that the seal at the front of the driveshaft is allowing transmission oil to migrate back to the differential and overfill it. Check the levels in both the transmission and differential. If the tranny is low and the differential is high, then you need to install an Oakie bushing, which will renew the seal. It's also possible that the vent on the differential (on top of the driver's side axle tube) is plugged. That will increase pressure inside the differential as it heats up, forcing oil out thru the axle seals and possibly other spots.