whine from rear axle

[steptoe]

its a 1989 250TD estate, 220k miles, the rearaxle whines, not very loud, no rumbles, oil level correct, its done it for a long time, doesn't seem to be getting louder, is it a case of changing the rear diff, or will it go on like this for a lot longer. Anyone had any experiance of this, what shall i do. Thanks;