i would get a second opinion that it is thepinion bearing.
is the seal leaking? it usually does if the outboard bearing is bad...

Does the growl get louder and softer while you accelerate and decell?

installing the diff mount incorrectly can cause the normal diferential noise to resonate like a guitar...

you can check the pinion bearing for noise with the rerar wheels turning while the car is in gear.
of course they are off the ground..

more diagnosois is needed before going off on a pinion bearing tangent..

__________________

79 240D my current toy
42 years a Diesel addict

240D sold
250SE sold
220D sold
280C sold
280S sold
300D (2) sold
300CD sold
300DT sold
300SD sold
380SL sold