would diff oil have anything to do with it? he said he read somewhere that might have something to do with it?
Yes, I'm aware of where that sensor is, but the ABS sensor is not even close to the diff housing. I'm not suggesting that you are wrong..just offering helpful and cheap advice, starting at the base level. ABS sensors are touchy things which mud, dirt and other crap can affect..sometimes just a simple, high pressure clean with a touch of detergent can cure a 'light on' problem. That is something I would be looking at and cleaning while the battery is disconnected..something else to be crossed off the 'check this list'. While you are down in that area, unplug the speed sensor (on top of the diff housing), give it a good clean, followed by a liberal dose of WD40 and reconnect. So far, all you spent is a bit of time..probably about an hour.