Probably two places to look -
If the shifter is mounted on the floor or on the dash away from the steering column that is likely where the NSS is located. They loosen up after a few 100-thousand cycles.
If the shifter is in the column, then your switch is probably on the top of the column, below the dash. They go bad after many years.
Some of the late model Allison's had the switch near the shift-arm on the transmission, usually two wires. You can do a check with an ohmmeter in park and neutral to see if you are getting a complete circuit. Easy replacement, too!
Good luck!
Lew