This problem has gradually crept up on me over the past two or three weeks. It's most noticeable on very smooth roads.
My 280E (W123) starts to vibrate - shake laterally - at just under 40mph. At just over 50mph the problem disappears. It's quite a pronounced sideways shake that does not appear to be coming from the front of the car, because the steering is not affected.
I'm thinking it's either badly unbalanced rear tyres (which need replacing soon anyway), bad rear alignment, or something to do with the driveline, such as the flex discs or driveshaft center bearing.
Any thoughts? Is this dangerous, or can I live with it for a bit? And if it's a driveline thing, how expensive are these problems to fix?