I had the same problem on my 2wd saph. Mate of a mate is a ford mechanic and gave me the test procedure. After checking the values stated i sussed it down to a faulty sensor which i duely got from a scrapped scorpio at a local breakers for pennies.
If you remove the plug from the ecu (you'll have to undo the securing screws of the ecu first) meter between the the 8 pins off to left-one probe in the top row, one probe in the corresponding lower row. you should see a resistance of between 900 and 1400 ohms.
This may show the fault up as a sensor. if they're all ok, there is another test you can do.
I'll try to find the paper work showing the pin numbers and axle position, as the numbers are on the plug if you remove the cover to expose the wires.
Hope this helps out