Then that is most likely the slave is not disengaging the clutch enough so input shaft is still spinning under a little power, so you have to force it to slow down by pushing/pulling harder on the stick shift this also wears out the syncro "gears".
Reverse has no syncro "gear" so it will grind if input shaft is still spinning.
There is a clutch fluid reservoir under the hood next to the Brake booster, make sure it isn't empty.
The clutch slave on the Rangers is ON the input shaft, so the engine and trans have to be separated to get to it, at that point the drive shaft is disconnected, the trans mount is off, the shifter is off, so...............
Drop the trans out and fix the OD.