You have a 5R55E in a 2001 Ranger 4.0l
Transmission is actually a 4-speed(4R55E) that the computer makes a 5-speed by turning on OD in 1st for a faux 2nd gear
1st, OD on = 2nd, 3rd, 4th, OD(5th)
If you "floor it" from a stop computer will usually skip "2nd"(no OD on) and shift to 3rd, just FYI
OD on/off button doesn't effect 2nd gear
Yes, if you have OD OFF and then turn it on on the highway it should shift to OD(5th), just 1 shift
Ford does make a specific Transfer Case fluid, XT-2-QDX, but yes Mercon V is fine to use