Mine was leaking, I pulled out the diff and resealed it when I was doing the Billetflow brace and MM diff bushings. Not hard but time comsuming. I used a gasket in mine (from the thunderbird SC diff) along with ultra gray and filled her up with fresh synthetic and friction modifier, I probably had 6 hours in the job by the time I was done.
Don't you only have to use friction modifier to break in a new diff?
I'm assuming you had to drop the diff to reseal the cover. Did you have to disconnect the upper/lower control arms to get the half shafts out of the diff?
