Swapping out the control box is the stupist thing you can do.

Most likely it is an ABS sensor or the wiring to the sensor.  You won't know
which sensor it is unless you get the car scanned.

I had ABS on my van go on about a year ago.  I got it scanned.  The problem
was the right rear ABS sensor.  A new sensor from the dealer cost $115.  It
came with the sensor and about a dozen feet of cable that snaked around all
all over under the vehicle.  I replaced everything.  The problem turned out
to be
a small break in the ABS sensor wires about halfway along that cable.  I
would
never have been able to find it unless I had taken that entire cable out.
That
was why the automaker supplied the sensor plus cable.  Sometimes their parts
designers aren't stupid.

Ted

To answer the questions:
Yes I have the owners manual and the Chiltons and the Haynes and the
TSB notices. It also says the lights comes on to test the lights.
Ford has also used Lucas charging systems in the past which ran
systems through idiot lights so systems failed when the lights burn
out.  When the antilock light goes out, does it really mean mean it
passed the test OR THAT IT FINISHED THE TEST???  The second time from
last I took a car to a Ford dealer, they fixed the problem and broke
two harness connectors doing it. An extra month in the shop.  The last
time I took a car to a Ford, I discovered that their highly trained
tech forgot to reconnect one of the rear brake adjusters (while going
down a mountain).  Since the repair is not under warranty why would I
want to go there?  Except to pay them $200 to replace the sensor and
have the problem come back a month later as listed in other messages.
But as you recommended I have taken it to two mechanics.  Both put it
on their diagnotic machines, returned it, and told me nothing found,
and don't stop in the rain unless I wanted to pay by the hour for them
to trouble shoot the system.  And I did take it to an "expert", my
brother-in-law who is a mechanic for a GM dealership and drives a
Bronco.  He can't get his antilock brake light to go out either. Oh by
the way, did I forgot to say that I am SAE certified. (So go back to
chatting about Toyota gas milage.)

Yes there is fluid in the master cylinder. I have never heard of
brakes locking up when they are out of fluid.  And the brake trouble
light is not coming on.

The codes were read with a snap-on reader.   It reads 2 and 3 digit
codes and displays sensors status which are repeated by the on-board
computer.

The '91 has RABS which only has one rotational sensor found on the
diff.  The sensor has been replaced twice.  However the cable may be
the problem. It would be simpler to test the cable from end to end
then pulling it out sections.  But that would mean finding the other
end of the cable (which is connected to the control module).  I have
found four modules behind the dash and none of their wiring matches
the sensor wiring nor what the wiring diagram shows the module's
wiring should be.  Since I don't want to figure out which is the
correct cable by running voltage though the sensor wiring because it
might damage the wiring on the hydrualic part of the system.  And
since the modules only have partial part numbers.  Would anyone know
what the antilock control module looks like?

ABS is a curse in the snow... my ecu needs to be replaced, and i wont
bother.. the ABS is useless to me