I just replaced my rear disc, which was cracked. It seemed to clear up a vibration at lower RPMS.

It does not look like the rear disk can catastrophically fail. The drive shaft has a hole that engages a pin on the rear differential. If the disk were to fail the shaft would spin, but not flail about causing damage. The front disc is a different matter though...

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine