Isn't that reference about a two speed rearend?
All tonnage Advance Design Series trucks used a banjo type axle housing.
Requiring a front carrier gasket and rear cover gasket.
Page 1 and
Page 2 There is one troubleshooting section for all four types of Axles.
See: 4-40 under
REAR AXLE AND SUSPENSION there is 3 troubleshooting pages.
Lubrication Leaks, I assume are referring to all four carriers.
Interestingly they don't mention rear cover leaks.
I think the leak here is where the torque Tube is pressed into the housing.
They where probably pressed and definitely riveted and maybe some type of sealant was applied. See
Rivets above Arrows 1 and 16 Could be leaking at that joint.
I wonder if something is cracked.
That is a possibility.