Strange noises from my rear!

Hello and Happy New Year to all.
Wonder if anyone can help with conformation of a noise.
It is a gentle deep tone under power which gets worse the faster i go, noticably from 55 ish upwards and is much worse when i back off the throttle. There is a gentle vibration that comes in at 55 mph too The car is a 4.0 Auto and i have not long changed the drive chain in the transfer box. I think it's the rear diff as although it has a new engine the car has done 217,000 miles.
Also, can i get the diff refurbed before it goes critical ? If anyone knows of a reputable non con merchant i am in the West Midlands near Dudley.

Any thoughts greatly appreciated.
Cheers, Nick

Whine could be the diff, the vibration may be worn prop UJ's

Hi.

I think i used to have one of those Cortina's. Is it a mark 3
Anyhoo i can't feel anything through the gear shift and the more i drive it the more i'm sure it's the rear diff.
Should i look/feel for anything in particular when inspecting the uj's. I'm assuming they should just have free fluid movment or would you just replace them. I don't mind spending the cash but the point is i don't want to if i don't have to.

Cheers, Nick.