87 560SL differential replacement questions

[rgofast]

My girlfriends '87 560SL has been making a low pitched whurring noise from the center rear of the car. It was doing this when I met her a year ago, so I dont know for how long. I suspected the differential, so she took it to her mechanic, actually two mechanics who said it was in all probabbility the differential. They quoted an astonomical price to replace it 4000.+ I have located a NOS differential for this car for 400., and plan on installing it myself within a week or two. I have mostly worked on US cars and Porsches, which I have done engine rebuilds etc. I feel that I can handle this job, but was wondering if there were any special tricks. Also I would like to make sure the diagnosis is right before I start. Is there any way to confirm the noise is comming from the differential, and not a wheel bearing, or drive shaft bearing. The noise does not change when cornering hard right or left. The noise coasts down with the cars speed, and is evident untill the car reaches about 5 MPH or so. The flex plate has been replaced, and the differential oil was checked, and I believe changed. The 911 uses the same basic type axle shafts and CV joints. Is there anything to it besides disconecting the axle shafts and drive shaft at the dif. placing a floor jack undernieth, then undoing the forward mounting bolts, and the mount in the rear, and lowering away?? Any special tools required? Looked like the rear mount that goes thru the mounting ring at the rear of the dif. requires a large torx tool or somthing.

Your help would be much appreciated. Also If I were to have purchased a new differential from a dealer, how much would I have to pay for the part.?

Thanks

Rod