5R55E was in all 4.0l Rangers 1997/8 and up, 2WD or 4WD
5R55W was first used in 1999 Fords and in Rangers starting in 2001/2
"Shift in the fly" means front and rear drive shafts have matching RPMs, which would always be the case in 2001-2011 4WDs because of Live Axle, always locked front hubs
In previous years the transfer case had/used an electric clutch to RPM match drive shafts
So all Ranger 4x4s had "shift on the fly"
Now manual transfer case Rangers, 2000 and earlier, would usually come with Manual locking hubs as well
The thinking here is that a driver would have the front hubs Locked, BEFORE selecting 4WD on the manual shifter, so RPMs would already be matched
Because there really is no advantage shifting transfer case to 4WD if front wheels are unlocked, lol