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.