when you do it...
(Assuming your disco axle is 24 spline)
You will probably have a 3 bolt Rotoflex coupling on the input to your rear diff.
To remove this...
Find a bolt that fits the hole in the nose in the middle of the flange.
Find a deep socket that is big enough to press on the flange around the nose.
Wind bolt in to pull out flange.
Then, unbolt the flange.
You will then need a 24 spline input flange, 4 bolt. You can get them. The chances are, you will also need the half inch or so spacer from the 4 bolt diff - this goes in under the flange and holds the bearing race in place. You DO need it.
Other than that, a pretty simple swap to be honest, me an a mate did mine in jus under a day, outside, in the rain, in bloody gale force wind.