Nice looking car!

The answer is YES, they should have both been replaced with a new pair. The question is now, what to do about it?

The only right thing to do is to now replace both old shocks with a new pair. Replacing one to simply match the brand and model to the other makes no sense since you will then have one old shock and one new one of the same make and model which, IMHO is no better than what you now have since once it is used the shock will have different characteristics than when new. So, either replace both with a fresh pair or just forget about it and drive.

Obviously, if it is going to gnaw at you as it would gnaw at me just install a brand new pair and know you've done your best to optimize the handling.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz