guys does anyone know what is causing this.
When I am slowing down there is a hell of a rattle from the rear do I need a new diff or is it the prop.
cheers.

As ever "it depends" ...

Check the rear prop UJ's for movement, the rear prop that it's greased and can move on the splines (It needs to come off to check this), the diff bearing - the flange shouldn't have any play other than a tiny amount on the diff splines, towards the Transfer box check the handbrake drum and shoes, again with the prop off you can more easily feel for play or see if anything is broken ... That's all a starter ...