Possibly like you, have had an unusual "clunk" when taking off from standing start on my
B200E which now has 244k miles. Particularly on uneven or adverse camber junctions.
Dealer say's nothing to worry about yet, as prop shaft has a joiner connection surrounded by rubber housing that wears down with age allowing prop to go out of true when under adverse taking off conditions like car not beind on flat ground.
This has been like this for last 70k miles & hasn't deteriorated in all that time!
They suggested replacing rubber bunnion for a cost of more than the cars worth! And after telling them to 'get stuffed' we all had a good laugh!

So check the prop shaft uj joint half way down to back axle.
Hope thats of some help.
Keith.