91 Nissan truck 5 speed transmission location for reverse lockout

The synchronizer for that gear could be bad . Will it go into reverse with the engine off ? https://en.wikipedia.org/wiki/Synchronizer

Define won't crank ? I'am assuming the key will turn to the start position ! When doing so does the starter spin the engine or not ? Trying to define your problem of won't crank . If when you turn the key to start position an the starter spins the engine over but doesn't start an run , this is a cranks no start condition . If nothing happens when you turn the key to start , this is a no crank no start condition . Your vehicle has a safety system , shift interlock , won't let it shift out of park if the brake pedal isn't pressed . Won't let the key turn off either . Electronics involved , locking solenoid's etc.... Floor or column shift ?
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the engine is running. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:
• The automatic transmission shift lock solenoid (serviced as the automatic transmission shift lock actuator), as well as the body control module (BCM) and the engine control module (ECM)/powertrain control module (PCM). The shift lock solenoid is located within the floor shift control assembly with vehicles equipped with floor shift.
• The BCM controls the voltage supply circuit of the shift lock control solenoid. The following conditions must be before the BCM will supply voltage to the shift lock control solenoid.
The BCM controls the voltage to the shift lock control solenoid through the shift lock control solenoid controlled voltage circuit. The following conditions must be met before the BCM will supply voltage to the shift lock solenoid:
• The ignition is in the ON position.
• The ECM/PCM sends an input via GMLAN serial data to the BCM indicating the transmission is in the PARK position.
• The BCM determines the brake pedal is applied according the brake pedal position.
Since the shift lock control solenoid is permanently grounded, the BCM supplies voltage to the automatic transmission shift lock control solenoid, mechanically unlocking the shift lever allowing the driver to move the shift lever out of the PARK position as the solenoid energizes. When the brake pedal is not applied, the BCM turns the control voltage output of the shift lock control solenoid OFF, de-energizing the shift lock control solenoid. The de-energized solenoid mechanically locks the shift lever in the PARK position.
During remote start operation, the BCM will de-energize the shift lock control circuit, locking the shift lever in the PARK position.

Plus the reverse lock out !
The reverse lockout system is a safety feature which prevents an inadvertent shift into reverse at speeds above 5 km/h (3 mph). The system consists of the following components:
• The engine control module (ECM)/powertrain control module (PCM)
• The reverse lockout solenoid
With the ignition ON, battery voltage is supplied directly to reverse lockout solenoid. At forward speeds above 5 km/h (3 mph), the ECM/PCM commands the reverse lockout solenoid ON by grounding the control circuit of the solenoid. This energizes the solenoid and mechanically prevents the shift lever from being moved into the REVERSE position. At forward speeds below 5 km/h (3 mph), the ECM/PCM commands the reverse lockout solenoid OFF, de-energizing the solenoid, which allows the shift lever to move into the REVERSE position.

Power Mode Master
This vehicles BCM functions as the power mode master (PMM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination the power mode that will be sent over the serial data circuits to the other modules that need this information, and so the PMM will activate relays and other direct outputs of the PMM as needed. Refer to Power Mode Description and Operation for a complete description of power mode functions.
You have no clue of the electronics in your vehicle , your best bet have it towed to the Buick dealer .

if the gear lever is into the top of the box, remove the turret and pivot ball and see why the shifter rails are not lining up
if it is a column shifter , check all linkages for adjustment or fallen off

Try rocking vehicle back and forth ( you need help) with pressure on shifter towards neutral

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replace all 3

i had a nissan pickup that would whine in all gears except 4th...it started out with 1st then reverse then went to the rest...solution was new transmission...or rebuild cause the syncronizers or the gears them selves are being chewed up...the new transmission was cheaper then a rebuild. or take it to somewhere that will do free diag on the transmission.

The speed sensor is located on the transmission towards the tail shaft. The housing that encloses the cable will attach on the side of the tranny. At this connection you will find the speed sensor right where it attaches to the tranny. There will be a couple of wires attached to the speed sensor.

check the transmission fluid level and condition

Reverse is not sycronized, this means the gears have to stop to mess and not grind, two thing cause this, being to quick to slam into reverse after clutch is depressed, a problem with the clutch.