There is a little wire that comes out the back of the differential, this is the speedo
signal cable.  There is a reluctor plate inside the diff, and sends pulses (hall effect)
to a frequency to voltage converter IC, basically, it is possible that this wire isn't
working well (bad connection somewhere) and this could be affecting the Anti-lock light to
be on, someone else posted something like this as well.

Check that wire from underneath to make sure it is in good shape, if it is, a more
thorough test would be required.  If you live anywhere near Toronto, I wouldn't suggest
Robert Motors.

Keep us posted with details, cheers.