I think that JDUB is refering to the device at each end of the shaft that centers the flex disc by sliding over the half inch diameter end of the trans output shaft or pinion shaft at the other end.
I don't know how often these sleeves fail, but I have seen every form of destruction from continued use of vibrating driveshafts. Probably the worse was one where the driveshaft got off the trans output shaft at 50mph. The driveshaft was then driven by the rear wheels until the car could be stopped. The driveshaft was destroyed (let alone the sleeve), the rear tailhousing was broken at the mount, and the crash profile sensor activated the airbag at 50mph. This was a 87 300e and the sensor is above the end of the trans on the tunnel inside the car.
It takes a special owner/operator to get into this situation.
Bosch Master, ASE Master, L1
33 years MB technician