The flexdisc is a flat, round piece of strong rubber at each end of the driveshaft. It serves to transfer the torque through the driveshaft while arresting any harsh impact.
I have had tires do all sorts of things to my 124 car. You can look at the flexdiscs and see if there is any cracking. But I would cast a very suspicious eye on your tires.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in