Ford says that Rangers with manual transmission, 2WD, or 4WD in 2WD mode, can be flat towed upto 55MPH, not sure that speed would be accurate since transmission and transfer case are being lubed at any speed, so I think its more of a Safety thing, i.e. Ford Don't want to get sued, lol
So its fine
Automatics can't be towed because they are lubed by their front pump which is only working when engine is running
Ford did have software they could add to electric shift 4x4s so the transfer case could be shifted to Neutral, but its been discontinued, this was mainly for 4WD automatics but could be used on manuals as well