Hi  I'm not a jeep specialist but have seen this on honda cars, what I have found was the wiring for the abs module was repaired by a body shop, it looked like a good connection but was not as I replaced the wiring harness and all was good, don't know if this helps but I would start with a good look at the abs module connectors and wiring,and check to see if the vehicle was in a accident. If in a accident then further investigation into the wiring harness near the accident area.