Am i missing a breather cap?

Tooljunkie said
A lot of gearboxes are filled to 3/4 to 1” below fill hole.

Many trucks run the vent up into engine bay,some clamp to a bellhousing bolt.

^^^^^^^This^^^^^^ Gear oils can have a pretty high expansion rate. Newer oils expand less when hot and also have better anti foaming agents.

I'd drain a pint out of it and see what happens. You may still need a baffle.