I also get a "clonk" sound in my '79 W123, similar to what you describe. I found the source to be the spider gear shaft. You can check for the source by rotating one wheel by hand and resisting rotation on the other wheel with your other hand. If you find a lot of play there, you could have the same problem as I do. Also check for play between each wheel and the driveshaft.
I cured my problem somewhat by switching to 80W140 gear oil. The "clonk" sound is still there, but much quieter.