Transfer case leak

I recently noticed that the bottom of my transfer case is soaked in oil. I have a 2008 Tacoma with 222,000 miles. Here are some pictures, it looks like it starts at the bump out at the top, looks like a cylinder on the side of the t-case. Any suggestions would be great!

 

This plug sits in the hole where the speedometer sensor used to be. Pull it out and replace the o-ring in there, that would be a good first step..

 

Dumb question have you ever actually check the oil level? With 200+k I don't think I would loose sleep over it.

 

I’m going to drain and refill this week, just wanted to solve a small problem while I’m at it. I’ve had the truck since 2018 153,000 miles and gave never changed it.

 

does anyone know the dimensions of this o ring? cant seem to find a cross reference for part numbers.

 

Not trying to be rude, but you don't have to scour the internet for a tiny little o-ring, and then burn a bunch of jet fuel flying it across the country. Pull it out, go down to literally any parts store near your house and they'll have an o-ring selection that you can match up