If you have a mechanical speedometer the speed sensor is a small device screwed to the bulkhead and the speedometer drive from the gearbox comes up to the sensor, then there is another speedometer cable from the sensor to the speedometer. In the past my 3.9 had stalling issues which were due to loss of the power to the heaters in the O2 sensors. I did have to change the O2 sensors at one point as one of them was getting lazy and causing one bank to run rich.
