I've got a noisy diff in my 2.0 DOHC Granada.
I get a nasty vibration under load between 50-65mph similar to a prop donut breaking up. Below 50 or above 70 it's quiet.

I've had the prop off and took to North West Propshafts who declared it perfectly fine. Drive shafts and CV joints move smoothly and just have a bit of play in the splines.
I've swapped the wheels round just in case it was that but no joy :-(

I've got to try the local breakers tomorrow but there's no 3.92:1 7" diffs on eBay. There's a couple of 7.5" for sale so I've asked them if they are sure they they are 7.5".

I'm assuming it's the bearings in the diff that need doing (I've changed the oil too) but I know that you need special tools to do the work. Any idea of the likely cost ?

Thanks