I had the same thing last August with my 2000 WJ. I went to the dealer & paid 45.00 to get the codes since you can only read them with the DRBIII or equivalent. A standard OBDII reader or the key method doesn't work for ABS. They said it was left front & right rear sensor and the abs motor was bad too. I went home & measured the sensors with an ohmmeter but I found you can't check these sensors that way, so then I disconnected the pos battery terminal then disconnected the connector from the ABS computer, made sure there were no recessed pins or broken wires, reconnected it, reconnected the battery. The light was out and the ABS has worked fine since then.
2000 WJ 4.0L 42RE 242 TC
180,081 miles & counting