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Is the axle assembly properly vented? If not, after the oil heats up the pressure will push it past the seal. My father just went through this. Not saying this is exactly your issue but it is free to check and an axle vent is super cheap.



Ah, I forgot to mention this in my previous post...yes, the axle housing has a brand new vent on it. I will double check today that it's open and vents freely though.

In my limited drivetrain testing I really wasn't able to heat it up...the seal starting leaking basically as soon as I brought up the RPM fast enough so start spinning the shaft at a higher speed. Hard to explain....but with the wheels up in the air (car on axle stands) and the rear end geared 3.91 I was getting about 2700 RPM...so maybe 40 MPH?