Differential noise while coasting

All,

I have an '82 Scrambler with the original AMC 20, 3.73 rear differential but
with one-piece axles. About 1000 miles ago during a long road trip, I
started hearing a rhythmic rubbing noise coming from the rear end when I
was coasted through parking lots. The rhythm of the noise was directly
proportional to the Jeep's speed. I isolated the noise to the rear
differential by removing the rear drive shaft. The noise only occurs when
I coast forward, not backward. Also, I do not hear the noise when the
differential is being driven by the drive shaft rather than the wheels
(although it's possible that the engine noise is masking it - the noise
isn't that loud).

The noise hasn't gotten significantly worse since I first noticed it, but
neither has it gone away.

The rear wheel bearings were re-packed 18 months ago and the differential
fluid changed at the same time. I see no obvious evidence of differential
fluid washing out the wheel bearings' grease, but I haven't taken it apart
yet.

Any hints on what this might be? If it needs a re-build, how soon will this
become a serious problem? Or is it something that can be fixed by simply
replacing a bearing?

Thanks in advance.
--
Michael White "To protect people from the effects of folly is to
fill the world with fools." -Herbert Spencer