91 300TE 4MATIC. Why is hydraulic fluid is coming out of the breather?

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Hydraulic fluid pumps out of the breather that is on top of the unit behind the transmission. It looks like it is a seperate unit between the transmission and transfer case. I might be wrong and it might be part of the transfer case. But does anyone know why hydraulic fliud would pump out of there? It drains the resivoir. I think it only happens at higher speeds. Please help!