Read up on rear subframe bushings. New bushings go a long way to keeping the front and rear axles on the same page.

My guess on the wear is either too much toe out or too much camber, either of which can contribute to wandering.

How much play is there in the steering wheel with the engine idling and wheels straight ahead? IIRC there shouldn't be more than an inch or even half an inch. Worn steering shaft couplings or looseness in the steering box can cause wandering, but you'd feel it through the need to correct your heading.

I take it you observe the swaying even when there aren't grooves in the surface.

Sixto
87 300D