Typically on these cars when the flex disc needs replacing so does everything else on the drive shaft, including the support, bearing, boot, etc. The joint should not have slid apart like that, it should have been tight with the giant 46mm nut/collar.

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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
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