Mercedes 420 sec rough gear change

[420sec]

Hi guys, I have mercedes 420 sec 1989, I have this car for nearly 2 years, but as I have only started using now more often I found that when auto box is changing gear (doesn't matter up or down) a sort of dead knock or clunk noise is heard coming from the back of the car. The knocking sound is not consistent sometimes (rarely) the gear change is smooth - mostly the knock is mild but sometimes can be quite loud and can send quite big shock wave through the car which I can feel through the drivers seat. Loudest knock happens in 30-50mph zone, as mentioned before doesn't matter if it's donwshift or upshift. I have lifted car up, and recognize few things: there were 2 small cracks on rear flex disc, one from one side and one from another (attached photo),there was oil leak from differential from both sides (attached photo) and it was a bit play in driveshaft. What was done already: changed gearbox oil and filter, checked when car was in running temperature, it's between low and max; tried adjusting gearbox modulator, was no difference, tried checking vacuum lines, found one that was disconnected completely, but when I put it back together made no difference. I've done all those things as I thought it might be problem with gearbox, but now I'm starting to think it might be something with flex disc that needs changing. I am no mechanic and any advice with this matter would be greatly appreciated.