While changing the lube in the front and rear differentials this weekend, I noticed that the rear had a plug but no gasket. I would have expected a crush washer or some sort of gasket. I tried to see if it was just stuck to the housing, but I couldn’t get one off. Shouldn’t there be one?
I put it back without one, and it’s not oozing, but …
My Ford explorer also didn’t have a gasket. But, it was put on at the factory with RTV. I scraped off all the ‘gorilla snot’ and replaced it with a real gasket from Fel-Pro for $6.00 at Autozone.
The plug has pipe threads which are tapered and seal when tight just like water and natural gas pipes.
I plan to find one for next time, but that’s a long time away. I’ll keep it watched for a while. I hate to drain it again to put a gasket in; it’s got $20 worth of gear oil in it 
Thanks for the reply.
Could be worse. My truck requires 75W-140 synthetic gear lube plus a limited slip additive. Best price I found was $67 just for lube and additive.
I wondered about that, but the underside of the head appears to seat against a finished surface on the housing. The front differential does have a short plug with what appears to be a copper gasket.
Nonetheless, it doesn’t appear to be seeping.