The 4R55E and 5R55E are the same transmissions, both are 4-speeds
Off-roaders with the A4LD trans(grandfather of the 4R/5R) high-jacked the 3-4(OD) shift solenoid to make the 4-speed a 6-speed
They would turn OD on in 1st to get a new ratio(gear) between 1st and 2nd, and also in 2nd to get a new ratio(gear) between 2nd and 3rd
The 2nd and 3rd in between ratio wasn't much of a difference, but the 1st and 2nd in between was good to have
So in 1997/98 4.0ls Ford tried it out, added that OD on shift to the Computer software, turns OD solenoid on while still in 1st, this gave a smoother(lower RPM) shift, so 1st +OD, then shift to 2nd, so 2nd is now actually 3rd, lol
So they made a 5-speed out of a 4-speed with no changes at all to the transmission, very clever these Americans