Chevrolet: gmc jimmy differential vent leak, vent tube, automatic overdrive

Question
i have a 1995 gmc jimmy 4x4.it has a vortec v6 and automatic overdrive.when driving after reaching operating temp. a small amount of fluid is expelled from a vent tube on the pass side rear of engone.it is directly over the exhaust manifold.makes lots of smoke for a short time.i am not sure what the vent goes to.it has a black cap.what causes this and what is the fix?thanx

Answer
John;
    Did you see what type of fluid it is. I would not worry about it , but if it keeps doing it , than there is a leak and it should be fixed. I would look to see where it is comming from. I have no idea of what fluid is comming out of it. keep a eye on it and if it does not stop than I would look into it.Can you tell me what fluid it is.--