I just went through the exact same thing. As it turns out, the threaded part of the plug hole INSIDE the diff cover was broken. So when I went to screw in the plug, it had nothing to grab on and it fell inside the diff. I have a Salisbury rear end, so after a few posts on here and some shopping around, my local rover 4x4 shop (lokeycrosslander) sourced me a new Dana 60 cover for $29. I installed it a couple of weeks ago and it works fine. It is a true Dana cover and the Dana name brand is stamped right on it. It is unbelievable how much heavier and better quality it is than the stock land rover cover that it replaced. The stock cover is a light, flemsy piece of shit. seriously, when you compare the two covers, you wouldn't even believe the difference.
"There is absolutely no excuse for the way I'm about to act."