Transmission fluid coming out of speedometer

I have an 86 Mustang GT, with a 5 speed manual transmission. The speedometer is cable driven--not electronic. I have transmission fluid coming out of my speedometer and getting all over the backside of my dashboard. Why/how could the trans fluid be traveling up the speedo cable? All the o-rings are present and the transmission is not overfilled. The problem only happens when I'm going interstate speed -- 65 and above.

Is the trans vent plugged? Path of least resistance and all.