Another vote for LubeLocker!!!
Let me tell ya'll a little sob story. After buying my cummins-with-a-dodge-wrapped-around-it, my Big Blue Chevy 454 K30 ended up sitting for most of the next couple of years. It's still my favorite of the two trucks, but even the venerable BBC 454 cannot compete with the cummins for daily economy.
Not long before parking it, I had gone through my (real) front Dana 60 and finished by refilling it with Mobil 1 Synthetic (it was just prior to my switch to Amsoil, too).
Being unable to buy a Dana 60 gasket anywhere locally, I used silicone from an unopened tube of "copper silicone gasket maker" (the orange stuff from Permatex). Mostly because it was what I had on hand and all silicone is created equal, right?
WRONG!!!
I applied it to an absolutely clean (I used brake cleaner) surface, let it skin real good, put the cover on and torqued the bolts, let it sit overnight, then filled the diff with Mobil 1 gear lube the next day.
The truck saw very few miles before being relegated to a spot in my yard for a couple years because of the dodge purchase.
One cold day, my wife's S10 Blazer battery was dead from leaving her rear hatch unlatched. So she somehow managed to climb WAY UP into that big ol' crewcab on 35" rubber and drove it to work. Even after sitting so long, it fired right up in near zero temps.
By the time she got off work, it had snowed several inches and was still coming down hard. She knew how to lock the hubs in and how to shift the NP205 into 4HI and wisely did so to drive the unplowed county road home.
When she came in the house, I asked her if the truck had given her any problems? "No. But it sure makes a horrible noise when it is in 4 wheel drive. "
despite the weather, I immediately ran outside and slid easily under the truck and began to inspect everything. The NEW front driveshaft moved up and down a couple of inches at the pinion... Yikes! :{
I got an oil pan and socket set and removed the front cover.
Not a single drop of oil came out; but individual rollers that were formerly part of my pinion bearings sure did...
There was a perfect orange silicone bead seal from where the "full" oil level HAD been, on up. From that point on down, there was not the tiniest shred of silicone on either the housing or the cover. The oil had
completely dissolved it while it sat parked for two years.

thus letting the oil seep ever-so-slowly out so I never saw a puddle under the truck sitting in the gravel/grass.
Lesson learned: NEVER EVER AGAIN will I trust silicone. And the ONLY silicone I will use anymore is "Ultra Black".
NEVER, EVER AGAIN will I buy a tube of that copper silicone crap.
It is not easy to ruin a true Dana 60 like that one, and all it really needed was new pinion bearings, but that requires tools and expertise I do not possess to set up. It was a very good excuse to install the new-style axle seals and a limited slip carrier, but I could have done without the expense at that time.
So if you CAN buy a gasket for rearend, ANY rearend, DO IT!! DO NOT TRUST SILICONE!!
In all fairness, one of the reasons I went away from Mobil 1 Synthetics to Amsoil was because of the propensity of Mobil 1 Synthetics to leak from engines and gearcases that did not leak before. Seriously. There is something in their products that finds or causes oil leaks past seals and gaskets. I am absolutely convinced of that.
My Dodge received the same Mobil-Delvac 1 Diesel Synthetic the original owner had always used. Despite it being babied, he had already replaced the rear main seal with under 60K miles on the truck.
My valve covers always leaked badly, too, even with new gaskets, until I switched to Amsoil. After a short time, my v. c. oil leaks disappeared.