Hi Jamie,
Its not absolutely necessary to remove the driveshaft from the car to replace the gaiter. However, in order to get the old one off and the new one on you have to remove the CV joint from the end of the shaft. Just before the end of the shaft there's a groove. Into the groove is placed a split ring. As the shaft is entered into the CV joint the conical chamfer compresses the split ring until it is the same diameter as the splines. This permits the splined end of the shaft to slide into the CV joint. Once the shaft is sufficiently far enough in to the CV joint there's another recessed groove in the joint. The ring expands into the groove in the joint and thus prevents the shaft from coming back out of the joint. They can be separated again because the groove in the joint also has a chamfer on the way out. However the chamfer is very steep and it takes a significant force on the joint to compress that split ring again.
I've done it before with a hammer on the inner core of the joint but it risks damaging the races where the ball bearings run. Citroen shows a special puller tool for separating them.
Alternatively, there's not much work involved in removing the shafts from the car. The near-side is very easy. It simply pulls out from the diff. The drivers side is not quite as easy because there's an intermediate bearing half way along the shaft. Two bolts with oddly shaped heads retain the bearing in the carrier. They are captive however and therefore you simply have to undo the two 11mm nuts on the near-side of the bolts. Once the nuts are removed, push the two bolts back about 10mm and rotate axially through 90 Deg to remove their heads from the shaped recess which serves to capture them. The drive shaft is then simply removed like the near-side one (unless the bearing sticks in the carrier, in which case you have to shock it out with a hammer). In both cases plug the resulting hole in the diff with a rag to keep the gearbox oil in until you are ready to re-insert the shaft. Check teh oil level when finished.
Hope that helps.
Cheers
noz
ps
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