""can the diode be reached just by removing the pan or is deeper inside the tranny""
The "diode" is simply a one way clutch, it is inside the trans and not your problem.
Proper diagnosis would be to install a 300 lb pressure guage at the mainline port in front of the neutral switch and see if the trans is produceing high line. If it is ,then you have to figure out why. If line is not high, then it probably has a"shift kit
in it.
Ford never puts "shift kits" in their transmissions, but someone else may have. This can only be determined by removeing the pan and some valve body components.