Does sound like a bearing, but it could be in the diff.

Does the noise change or go away in a sweeping turn (say, an exit ramp?). This is typical for hub bearing failure.
After a drive, one tire might feel notably hotter than the other.
Thirdly, one of the snow tires you removed would probably show an unusual wear pattern- possibly inside edge wear, and perhaps some cupping.

Are you sure it is from the rear?
It can be hard to localize a noise underway.

Best of luck.

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1986 300E 5-Speed 240k mi.