On mine (a '97 Rodeo) it turned out to be the sensor plug itself - actually the socket that the sensor plugged into. The contacts were dirty, so I cleaned them out with WD40 and pipe cleaners, and even with a new sensor they didn't make good contact (wiggling the plug caused intermittent connection). So I just used needlenose pliers to twist the male pins on the new sensor about a quarter turn each (maybe a little less) and carefully re-inserted it, being careful not to bend/crumple the pins in case I twisted them too much to fit into the female sockets. That seemed to do the trick, and now the problem has been gone for a couple weeks and it passed inspection with flying colors
I did also notice some of the little retaining clamps that held the cable in place near where the front side of the leaf spring mounts to the frame were rusted off, so I re-fastened the wire in those spots with zip ties to prevent it from getting hung on road debris (especially when four-wheeling). It's important to NOT secure it too tight at the spot where it has to bend to accomodate leaf spring motion, as this may cause enough cyclic motion to eventually fracture it (ref: previous posts in this thread). I left it loose enough that the bending of the wire is spread over a long section of the wire rather than in one small place.
Peace
EDIT: My original sensor was probably still good until I touched it. Someone recommended cleaning the inside face of it with WD40 and a toothpick, to get the grime out of the little slots. I couldn't get it out with a toothpick, so I used a needle and wound up digging out copper - OOPS!!! So I ruined it by being stupid. Be careful if you try to clean the face of it - stick with a toothpick, and don't dig too deeply. BTW, unless it's metallic debris, it doesn't even affect it anyway, because the sensor is simply a magnetic field detector (hall effect sensor) and non-metallic matter doesn't really affect magnetic fields.