Front Diff Leak at Sensor above Fill Plug

So I changed my Diff and T-case oil @ 15K. Saw a drip this weekend in the driveway and crawled under the truck to see what it was. Come to find out it is weeping from the thing circled in red below.

I only filled the diff to the bottom of the fill hole with a trickle coming out.

So my questions are:

1) What the hell is the thing in red below? I had 1st and 2nd Gen's and neither had such an item. Looks to me to be a sensor of some sort?
2) Is there an o-ring behind this thing that could be leaking?
3) Anyone else seen these leak?


I double checked the level of the oil yesterday. All is good. I also checked the breather hose. All is good there too.


I was just at the dealer for the air dam motor error and tsb to have the fuel gauge software update. I just hate to bring it back for this stupid thing.

Crazy part, it never leaked until I changed the diff oil...Maybe these new trucks are sensitive to the oil level?