What's leaking here?

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I think that's the 4wd actuator control box on the transfer case.

 

The transfer case hold fluid along with the transmission could be one or the other but yeah that actuator is most likely your problem. How to fix it I am not so sure as I am not a mechanic and only know from whuat I know on here. I even surprised myself knowing what part that was. It has been discussed on here quite a few times.
Edit maybe @glorifiedwelder or @Speed Freek could give you better input
@dwd1985 here is a thread
https://www.tacomaworld.com/threads/007-toyota-tacoma-is-stuck-in-4wd-h4.327502/

 

Maybe look here:

https://www.tacomaworld.com/threads/transfer-case-actuator-problems.515051/

and here:

https://www.tacomaworld.com/threads/transfer-case-actuator.468541/

Hopefully this helps.

 

OP, a couple of thoughts. There is some grease inside the T.case actuator and some of the oil from the grease may be separating and a little leaking out the seam. More likely its shift rod enters the T.case and passes through a seal that could be nicked, dried up, etc. allowing a bit the gear oil to come out and into the actuator. That actuator is a bear to get out as there are two c-clips inside the T.case that have to be removed generally necessitating splitting the case. Way to go Toyota, NOT! I have read that a few have removed it though by opening the actuator and leaving the rod in place. Warning: the actuator is tough to reclock and Toy has no procedure. Rather, I have some diagnostics for it and if but am too lazy now to go find. If you PM me it will remind me later.