Why does my 18 speed roadranger neutralize even when selected in gear

I'm assuming your refering to a Chevy S10? There should be a small gear shift lever next to the standard gearshifter marked as 2HI, 4HI, Neutral, 4LO. This is your 2wd/4wd shifter. 2HI is for normal everyday driving as it is only powering 2 wheels for better MPG. 4HI is the same gearing as 2HI but powering all four wheels. This is your 'normal' 4WD setting. 4LO is a set of reduction gears designed for serious 'Off Road' conditions and should not be used on public highways. Your top speed will be Very low in this setting and the engine will be maxed out at full revs to get it.
DO NOT leave this lever in the Neutral position without the park brake ON. This setting completely disconnects the transmission from the wheels regardless of the gear you have selected so it will roll even if you select Park.

try the dealer spares department or try an accredited transmission repair shop for what you want
if it is a genuine isuzu gear box then parts may only be available form the manufacturer but if it is a roadranger or spicer box or allison automatic that may increase your possibilities of supply

P=park gear position (no gear selected)
R=reverse gear position
N=neutral gear position (no gear selected)
D=top gear
2=fixed highest gear (used for speed when climbing an average incline and to control speed on decent)
1=fixed lowest gear (used for climbing steep incline or descending slowly)
Note-: Never engage Reverse gear when traveling forwards and never put into Park when moving forward or backwards, also towing most autos should be done no more than 10mph, fast towing can damage the gearbox, also make sure gear is in Neutral when towing.

A 1955 Ford would have a Ford-o-matic transmission which has three speeds forward. The transmission starts out in second gear unless you shift into low.
I am not aware of these transmissions having a neutral safety switch.
The 3 speed automatic transmissions that started in low and had a downshift to second under load were first used in the mid 1960s.

YEP SHIFT LINKAGE LOOSE OR WORN NEED ADJUSTMENT.ADJUST THE SHIFT LINKAGE BY POSITION THE SHIFT LEVER IN NEUTRAL .REMOVE THE REAR TUNNEL COVER IN FRONT OF THE REAR SEAT.PULL THE RUBBER DUST COVER FORWARD ON THE SHIFT ROD.LOOSEN THE CLAMP BOLT ON THE SHIFT ROD.MOVE THE TRANSMISSION SELECTOR SHAFT ALL THE ITS LEFT STOP,KEEPING IT IN NEUTRAL.WITH THE TRANSMISSION STILL IN NEUTRAL . MOVE THE GEARSHIFT ROD TO THE RIGHT TO ITS STOP.TIGHTEN THE CLAMP BOLT TO 18 FT LBS.TEST THE SHIFT LEVER PLAY SHOULD BE THE SAME IN ALL GEARS IN ALL DIRECTIONS.

indicates a vacuum leak possibly from a brake booster failure
when in park the idle is good but when you select gears or slowing down you have your foot on the brake

faulty contacts in neutral switch replace with new unit

I assume it's a manual transmission.
If the car will not roll backward or forward in neutral (with or without the clutch engaged) the transmission is most likely locked in two gears at the same time.
This may happen if gear selectors are excessively worn. If you are able to examine the gear selection levers external to the gearbox, ensure they are in the 'neutral' (no gear selected) position when the gear lever is in Neutral. If not in correct position, you may be able to manually reset the levers to correct neutral position and thereby unlock the gearbox.
In any case, if it's happened once, it will happen again, so get the gearbox and gear selection mechanism checked for wear or breakage.

check your inhibitors switch or the shifting cable.