Reading these posts re flex discs, I decided to replace them when doing the Centre carrier bearing (from Needswings) as the car now has 99000Km. Got Dorman flex discs from a supplier,( PN # 935-403) but upon comparison with the existing MB parts, they are NOT the same. The holes match up but the originals (MB # 210 411 0415 stamping) have protruding metal bushings that fit into the driveshaft on one side, and into the yoke on the other. Thus the MB bushings are 3mm longer than the Dorman bushings which sit even with the rubber. There is no way I am putting these on the car. The front and rear flex discs are exactly the same on my manual transmission 2004 xf. Mine show no signs of cracks, but feel rigid.
The Flex discs can be replaced fairly easily if you loosen the carrier bearing holder to allow some linear travel of the driveshaft.
My carrier bearing was absolute toast, the rubber had disintegrated. The only manifestation was a clunk when taking off in first gear, otherwise no symptoms. Glad I found it before a catastrophy. Only the rear half of the driveshaft has to come out to service the carrier bearing. That is good because otherwise the exhaust would have to be jiggered out of the way. I only let the heat shields droop down, and removed one little plate near the carrier bearing. The rear driveshaft and carrier bearing slips nicely down towards the rear. The front half of the driveshaft is trapped by the manual shift brackets not allowing passage of the yoke when trying exiting towards the rear. The exhaust prevents exit towards the front. I just marked the alignment, and left it in place. Getting the right parts is far more difficult and expensive in Canada. Between exchange rates and Shipping, we get screwed. I got some flex disc part numbers from this forum and I will try again to get them from the supplier. As for the Amazon parts, they have a dizzying number of different ones. With a little digging I found the exact MB part number listed from a Chinese source but I can't really tell from the photos if they have the protruding bushings. If anyone has had success in getting these exact right ones from Amazon, I would appreciate confirmation of fitment. When I am all done and back together, I will follow up with the final results and sources for general information and for my Canadian compatriots.