It sounds like you are talking about a pressure reducing valve. I would be very surprised if it is truly discontinued. There was a recall on them a few years back (I don't remember which model years were involved).
During the recalls I learned that the ABS cars were not part of the recall and that there was a different part number for the ABS systems vs the non-ABS systems. If my memory serves me correctly, the rear disc systems used the same part number as the rear drum systems.
If yours was part of the recall, you may see if the recall was already done or not. If yours is involved and the recall has not been done, you should be able to get Ford to do the recall and replace the valve.
If the recall has been done and whoever installed the new part cross threaded it, I would try very hard to get Ford to pay for the new part, but it may be wishful thinking at this point.
