What plug is this and how do I repair the leak coming from it?

It's for a speed sensor that these trucks do not have.
There is an O-ring under the plug.

Take the plug out, replace the O-ring and DONE.

 

Looks like this

 

Looks like its a "thumb tab" so it should only be hand tight when it threads(?), pushes(?) back in?

 

Awesome! Thank you!

 

It's held in by the 10mm bolt above it in your picture.

 

Should I expect fluid to come out? Was told elsewhere that if fluid does come out then the transfer case was way over filled at some point.

 

There shouldn't be much come out, if any.
The fill plug is lower than than that.
So you should be fine. But you still need to check the level.
As obviously, you leaked some out.