My 300SD has about 280k as well, and in spite of many folks assertions that the diffs almost never wear out, mine is. Howls and clunks something fierce. Just had the trans fluid flushed and changed and now that the shifts are much firmer, it clunks every shift, unless you use lots of throttle. My indy checked the mounts and said they're fine, that the ring and pinion are worn, but that it can probably go quite a ways the way it is. Eventually, I'll try and find a good used diff to swap in.
That is what I did, because I lacked the time to try tightening the preload on the diff. Maybe not worth it if you have to pay for the job, but DIY you could consider trying to adjust it. The noisy diff I took out of the 240D looked fine inside, but was loose on the pinion and as a result, noisy. I believe it would have driven fine for years, howling and complaining every step of the way. Much like many people we may know! -- Jim
