My 99 c280 had a very annoying and persistent vibration at 58 - 65 MPH (it smoothed out a bit above 65). This fix may apply if you have aftermarket wheels.

It turned out I was missing two of the original four HUB-CENTRIC rings, they fit into aftermarket wheels and provide proper centering as the wheel is snugged up. They don't look as important as they are, as they are plastic and look like the outer ring of a mason-jar lid. They shim the wheel to fit snugly with the center axle hub.

I bought a fresh set for $9.99 including shipping - they come in a wide variety of sizes depending on wheel type but all have a 66.56 inside diameter which fits tight to Benz hubs.

Thought I'd better post this as I've chased this problem for over 18 months and had about resigned myself to buying new wheels.