Chances are the fluid changes will quieten the diff down, as you're doing it anyway i'd try that first. The prop centre bearing is usualy a whine or rumble that varies its pitch with speed and seems to come from just behind you under the floor, which is more or less where it is anyway.
Could also be worn universal joints, worth checking those as well.
A useful little tip for refilling the back axle is to remove the load space floor (i'm guessing you have an estate/Kombi) which is only 3 nuts, remove the fuel tank sender/pump unit access panel from the floor (4 screws) and feed a funnel with a piece of hose into the space, crawl under and push the end of the hose into the fill hole on the diff then you can lay inside and pour the oil into the funnel.
Gives you an idea!
