Take the propshaft out marking the flange on prop and diff flange with tippex/file mark for re-assembly later on. Prise out the old seal with a large screw driver. The end of the g/box main shaft does stick out slightly from the gearbox so take care not to damage the splines. Carefully drive the new seal home using a large peice of tube or socket on the very outer edge of the seal. If you jack up the rear of the car as high as you can and support it with axle stands etc, you will also stop the g/box oil escaping whilst you do it.
As a further thought, just check the end of the propshaft, the bit that goes into the g/box that it hasn’t got any excessive grooving/rusting where the seal runs as this too will cause it to leak and changing the seal would be a lost cause in this case.
