drain the axle, unbolt the half shaft flanges and retract the halfshafts, unbolt the prop shaft from the diff, undo the ring of nuts holding the diff in place remove the diff
find a landrover breakers, then pop down and get a second hand diff, its cheaper and easier than getting one off of ebay, any rangerover or discovery shoud do the job, i don'tthink they changed the ratio between coming out and the last of the classics and disco 1 uses basicly the same platform
its a pretty easy job, the one thing i will say is replace your propshaft bolts/nuts every time you remove them, they cost just over 20p each and for a couple of quid a time it saves sooooooo much agro when one rounds off and you have to lay under there and saw through them