While not MB experienced, you are describing exactly what I experienced in my 944. Ring and pinion is the weakest link in those cars and it had some when I bought it with 43,000 on it. By 150,000 the slack got to the point where it was almost like an on/off switch when applying the power. It did teach me excellent rpm matching when changing gears though. The problem did take away a lot of the fun towards the end. Good thing my next door neighbor, the factory trained Porsche mechanic messed up a timing belt change and blew 7 of the 8 valves when they greeted their repsective pistons. So, I'll never know the joy of driving the rebuilt rear end.