Mercedes-Benz: W123 rear tire uneven wear

Does the W123 (1982) have something to toe in or out the rear wheels, or are
one of my shocks bad? I've had a recent alignment and it's still wearing
very badly on 1 rear tire.

The rear shocks have nothing to do with the rear alignment. Your car has
worn bushings back there that allow a wheel to shift back and forth
making the rear no longer square to the road, thus the odd rear tire
wear.

Technically, there is a bit of rear toe-in that's created by the car's
ride height and the geometry of the rear suspension. There's NO rear
toe-in adjustment, only a height adjustment via spring pads.

So have the rear control arm bushings checked.