I'm always sorry to hear about someone's MB "on the injured list".
I may be able to offer some assistance one way or another...The flex disk would probably make such a "thump" if it let go completely, but I would think that you would still feel/hear the tranny shifting gears when you move the lever. Hmm...I've never had that happen personally, so I can't offer any more than that...
However...If your transmission is broken and is going to be big $$$ to repair, I have a good tranny from a 78 300D for sale. I'm in Atlanta, GA...don't know if you're close to me or not, but it's really one of the last significant items left on this car I've been parting out, and I'd love to see it go to someone who needs it, and then I can have the chassis of the car towed away for good.
I hope your problems aren't that serious, but if you need it, let me know.