due to the "unique" way that ford built sierra's, some are easier to change than others
what you want to do is lift the rear of the car up and put some axle stands under the beam
then put a trolley jack under the diff to support it when you undo the bolts
get the arb bolts out first to get it dropped out of the way so you have more clearance
then undo the actual diff bolts, these may be a little tight so you may need big muscles/power bar to shift them
then use at least a 12" extension to undo the 4 13mm bolts and then lower the diff out of the way
the rubber bush/bung thing is there to stop the fuel tank splitting if you are rear ended onto the diff casing and it has been known to be just a little troublesome to remove
they are very easy to change but i'm not sure if you need to loctite the diff bolts back in again