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I have a 2018 Ram 3500 that had the same issue. The dealer said it was fine so I inspected it myself and I also pressure tested it. This is what I found. Notice the tiny seam where the nut touches the plate? its a paper thin gap. Notice that the nut is just tacked on? Not an issue at all as long as your rubber washer is doing its job. When I pressure tested this I found out that there was blow by around the nut and my rubber washer was failing on one side. I replaced the washer with a larger one and I have had zero issues since. Not saying that this is the same issue fix for you but it worked for me. It took me a while to figure it out because logically I checked my axle/bearing and seals first, second my vent when I found nothing I did the pressure test. In order to do the pressure test I used a flat piece of 1/8" sheet metal and gasket maker to fasten and seal the diff cover. Then I welded an air nipple for a compressor hose to the back. After it cured I filled it with a light 5W30 engine oil for the test(because its cheap lol).