I'm interested to know how you determined that the diff is making the noise ONLY at 70 MPH.
Did you jack it up and spin up to 70 ?
Had a lady bring her coupe (90,000 miles ) by last year for me to have a look at and I would have sworn the diff was toast.
I jacked it up on the rear and spun it up all the way to 70 and there was NO noise.
It was her worn out tires, she got a new set of Hankooks installed and it was quiet as a mouse.