1999 Dodge Durango. ABS/Brake dash light on, need help testing

So Ill give as much info as possible.  My dash ABS and BRAKE lights are
on. I have just replaced my wheel speed sensor on the rear diff and no
change. I am not sure how to test the sensor but I took the old one,
plugged my multimeter into it and waved a knife infront of the sensor and
the meter is jumping around, I assume this means the old one was working
as well.
I have had the durango for a couple years.  I have not really slammed the
brakes to see if the durango will skid or not to see if the ABS is totally out.   
the truck seems to brake normally though. BUT.... There were a few times
here and there where after a rainy night, I would get in the truck and pull
out the driveway, and even the most gentle slight touch on the brake pedal
would cause the wheels to lock up and me to skid, but only at very low
speed,  in a parking lot etc but would not lock up at higher speed.  This
would happen for a short period then go away.  Has not happened for
about a year.    I am trying to figure out what the issue is.  Also I do not
have any Gear shifting or Speedometer issues, they are fine.    If anyone
knows all possible causes and most importatantly how to test for where the
issue is myself I would greatly appreciate it. I have a multimeter so if you
know how to test the wiring with it or other ways to test/eliminate possible
issues and narrow down the issue that would be awesome.  I can not afford
to take it to a mechanic to do it for me, I like fixing my own stuff just need
some guidance from those with the know how.  Just tell me how to start
testing the different sensors, wires etc.  Thanks.   ALSO.. It is a 1999 4WD,
5.9 V8 Magnum.