The most common causes of fluid leaking out of the vent from either a diff or an auto trans are often the same.
Old fluid foaming. Change fluid.
Overfilled. Reduce fluid level to the correct level.
Overheating. This one can be a bit tougher, but in the case of an auto trans it could be that it is overloaded (too big of a trailer of something similar).
Start by checking the fluid level and condition.
