OK, I know the model, to be honest, the motion sensors aren't best suited to 4 wheel vehicles for the reason you are having. This model has the micro level switch in the unit, so not as easy as just to disconnect it, you can omit this señor when arming, but that means doing it every time. Have you put the sensor to the lowest level? You can do this putting into the low sensivity mode and then you have 4 lower settings.
Its in your instructions, but basically, arm the device, then switch ignition on, wait around 15 seconds , led goes out, push button 2 for low mode, then watch for the light for it to flash low, then turn ignition off.
If you need more info, let me know.