I have a 1999 F250 7.3 Diesel and a problem not even the dealership can figure out.

Rear differential is spitting gear oil out through the breather tube which is up on the frame. My first guess....over filled. Not the issue. I have an after market differential which has a dipstick and the fluid levels are fine. I am running out of ideas even called shops that specialize in rear ends...no answer. Does anyone have any ideas?

R/s