Electronically controlled Transfer Case?
It MIGHT be towable with a "neutral kit." A misnomer because the "Kit" consists of a dealer re-flash of the PCM, instructions for a routine to cause the PCM to put the electronic transfer case into Neutral, and the hardware part is an indicator light that confirms Neutral.
The "Kit" is no longer available, but the re-flash is, the instructions can still be found, and the Indicator can be gotten at Radio Shack.
This only works on SOME 4WD Fords. Not on any "full time" or AWD ones. Most of the ones I've heard of being modded this way are Explorers, so it might depend on the exact power train in the Ranger.