2006 Mercedes-Benz E-Class - Back end issues

My back end whines really loud or sounds like road noise after relaxing tires and also alignment

Relaxing? Or replacing? Not sure what you mean

Probably “replacing.” I suspect autocorrect gone wrong. Anyway, tires with different tread can make a different sound when you’re driving. Beyond that, it’s possible the differential is making some noise. The OP can have it checked out for peace of mind.

If the tires are a change, have them mounted up front so you listen for the noise to move. You want to target or eliminate the tires. Tires are cheap compared to the rear differential.

+1
Additionally, the rear wheel bearings could be the source of the noise.

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That would also be my assumption