hi there can someone please tell me how to check the diff oil on a land rover discovery and how to top up do I need any special tools. Also does anyone know where I can get a free service manual from online. Thanks

its easy - look at the Rave manual - do a search.

To check the front and rear diffs, just undo the plug on the front of the diff and see if any oil comes out, the oil should just be at the same level as the bottom of the hole. Easy way to check is to use the 90deg part of a small allen key as a small dip stick.
To fill use a small funnel or some of the small bottles come with a tube, fill up until the oil just starts to come out.