Well, I suppose it could be a bearing in that case.
That it goes away under 'driven' load may support the bearing theory.
A number of folks have posted with rear bearing failures.
Cupping is where you have 'spots' of wear around the circumference of the tire, as opposed to an even band of wear all around. I have found this to be a common symptom of a bearing that has been dragging for too long.
I imagine the inner edge would be the place to look in this case.
Perhaps you could locate the problem by spinning the wheel with your hands.
1986 300E 5-Speed 240k mi.