You need to replace the vent tube that should be attached to the bolt holding the brake hose splitter to the axle housing.
Not having one can allow water to enter, and if the hollow bolt gets clogged your axle will force gear oil into the brake drums when it warms up in use.
It looks like you have a build tag attached on the left side of the diff cover, but it's hard to tell on my phone.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.