In the last two weeks or so, my 1997 E36 has started to "wag it's tail."

From about 15-20 mph, the car wobbles...not very technical, I know, but
that's the best way to describe it.  It feels like one of the wheels is
eccentric, like those little kids toys where the center of the wheel isn't
where the axel is mounted.  There is very limited feedback through the
steering wheel, so I suspect the back end, but I'm not positive.

It appears to be some kind of resonance between the drivetrain and the
suspension, since it goes away above 20 mph, comes back around 35-40, then
is gone until you get up over 65.  It gets really bad at 80+.  It scales
with wheel speed, not engine RPM, so I know it's something downstream of the
transmission.

At first I thought I'd thrown a balance weight off one of the wheels, but
that shouldn't have so pronounced an effect at low speeds (I think).  I
haven't had any collisions or bumps with anything that might have bent
anything.  The only thing that's a little off with the car at the moment is
my rear tires are overdue for replacement (they're starting to look like
racing slicks), but it's uniform wear without any flat spots or anything
like that.

Any ideas?  Wheel bearing?

Thanks,
Tom.