a few things to check, first is the rear abs sensor, its the one on the differential. its kinda hard to do on the 87, but that would cause the abs light to come on. on 92 and up trucks the same sensor is used for the speedo, so its obvious when its bad. second would be the RABS module itself, which could cause the pedal to go down, as it is what actually bleeds off pressure to activate the abs. third thing that will turn on the abs light is low brake fluid. another possibility on the low pedal is the master cylinder itself.