Originally Posted by zonefive

I just purchased a 2012 E70 3.5 XDrive with 70,000 miles, and after I replaced the horrendous run-flat tires I was able to hear a drivetrain whine above 20 miles per hour.

Took it to the mechanic and the rear diff was failing. Almost impossible for him to turn. He took it apart and said the spider gear is bad. All kinds of metal in the oil.

Sourcing a replacement diff from a wrecked car with 14,000 miles on the clock. Total repair cost $1,600 USD. Adding in another $2,600 for deferred maintenance (tires, coolant flush, brake fluid flush, power steering fluid flush, transmission and differentials fluid/filter change, plugs, new battery) and the car should be up to snuff for a while.

Car is in very good shape, doesn't appear to have been abused in any way, the differential may have worked hard in some snowy weather (car is from Chicago), otherwise there's no reason that the diff should have failed. Mine if a post-facelift, is everyone sure that they made changes in the diff when they facelifted the E70?