Clearly, replacing both makes sense.

Another option suggested, and which I am considering, is cutting the spring on the passenger side to match the length of the one on the driver side.

Right now there is no change in ride height because the "new" end of the broken spring has not rotated to where the original end was. (I suspect that is also the reason for the dreadful groaning and creaking.) If/when that rotation occurs there should then be a discernible lowering of height on the driver's side, and it was suggested as an option to cut the passenger side to match, which would cost me about the same in labor but about $200 less in parts.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black