The P1393 error on the VX indicates a voltage condition from the G-Sensor which determines whether or not you're bouncing around off-road so it can tell if what the VSS and crank-angle sensors are telling the ECU is true.

The G-sensor is located in the center console directly underneath the armrest storage area and behind the e-brake pivot. Remove your center console (real easy if you take the seats out which is also easy) and trace the wiring from the G-sensor forward to make sure the connectors are properly seated and that there's no damage to the wiring. If you've got electronic skills and test equipment you may be able to diagnose the component yourself. Here's a basic description of the G-sensor:

Circuit Description
The Gravity (G) sensor is a vertical low-g (gravity) acceleration sensor. By sensing vertical acceleration caused by bumps or potholes in the road, the powertrain control module (PCM) can determine if the changes in crank- shaft speed are due to engine misfire or are driveline induced. If the G sensor detects a rough road condition, the PCM misfire detection diagnostic will be de-activated. The G sensor at rest output should be between 2.35 - 2.65 volts (+1G). During a rough road condition, the voltage output can vary between 0.5 volt (-1G) and 4.5 volts (+3G).

Conditions for Setting the DTC

Engine running.
No vehicle speed.
G sensor signal indicates greater than 2.5 volts (+1G) or less than 1.5 volts (0G) for one minute.
The G-sensor is present in many Isuzu 4X4 vehicles so you should be able to locate one either new or from a breaker.