You shouldn't be buying the parts if you can't install them.

Hopefully you still have the original modulator.  If so, replace the
aftermarket with it, then go to the Ford dealer and have them
replace the modulator.  Otherwise, what I would suggest is you
go to a wrecking yard, find your
model truck with an original Ford modulator, get that, have your
tranny shop replace it, then see if it works.  If the wrecking yard
modulator is also leaking, but the shifting goes back to normal,
then you can take it to the Ford dealer and tell them the truck
works fine, and your losing fluid, and you called a tranny shop
who said the modulator is probably leaking and to take it to a
Ford dealer to replace since they can't get that particular modulator.
Naturally don't mention you already
tried aftermarket or that it's a wrecking yard modulator otherwise
your giving them something to blame you on if they put in a
Ford modulator that doesen't work.

What probably is going on is that Niehoff when they made
the aftermarket substitution, they made a mistake with your
model and year.  As a result, I would guess most if not all
aftermarket modulators are going to be wrong for your truck.
This is kind of a lost cause because it's possible that Ford themselves
made an error in their documentation.

If you go to a dealer and the truck is shifting fine, the dealer
then puts in a genuine Ford replacement and the shifting goes
cockeyed, then it is their problem, and they will have to make
the phone calls to Dearborne to get an engineer on the line to
correct the documentation and tell them what modulator to use.
For sure your not going to be able
to do that, nor will any aftermarket tranny shop.  And for sure
Niehoff will likely not be able to assist either - although a phone
call to them, while a very long shot, might work.

Your experience is one that goes along with the territory of
buying aftermarket parts.  Aftermarket manufacturers will often
come across the situation where the auto manufacturer makes
an unexplained change in a part - maybe only for 1 year - and
it's not cost-effective for the aftermarket manufacturer to
manufacture a special part for only that year.  So they will take
a similar part and make a substitution.  Sometimes that works,
sometimes not.  The aftermarket manufacturer will simply
look at the returns and if they see a high number of returns on
a substitution, they might then pull it from their docs.  Or they
might not.

Ted