I still like to use gaskets for most stuff, usually with Permatex #2 on both sides.

However, crenwelge is right, differentials for years now have been assembled at the factory with silicone. I took a Tour of the Dana axle assembly plant near me a few years ago, they used a black silicone on the covers.
The new Rockwell/Meritor axle I put in a truck last year said in the service manual to use silicone to seal the third member to the housing. I did and it worked just fine.

I Don't care for how silicone looks when it oozes out, so I wipe the extra off with lacquer thinner, or let it set and hit it with the wire brush later.

There is still a chance you will find gaskets, and if not you could always cut some out.
Have you tried http://www.olsonsgaskets.com/ ??

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-