The 5R will only shift through four gears when used with the 4R PCM. It will skip "2nd" so it will be just like the 4R. But the PCM for the 5R is not plug in play in a Gen III Ranger, so you will still have the advantage of the heavier trans with bearings, but will still only be a 4 speed.
The neutral safety switch is the switch that makes it so the truck can only start when its out of gear.