right rear wheel locked up ( abs )

wheel is locked up but not by the brakes.  when jacked up the wheel will rotate
fwd and back  a  slight amount (1 inch) at the tires surface. I can hear  and feel
it stop both ways. as it moves i can see that the right shaft is also rotating a
slight amount at the center gearbox.   (note that applying any brake will stop
this slight movement)