Q: My truck won't start, battery is good, alternator is good, don't know if its starter, was told might have to change neutral lock.

You will need to test the small yellow wire with a blue stripe that goes to the starter and powers it when the ignition switch is turned to start. If the the wire checks out with power then the problem is with the starter. If it does not get power, test the start relay in the junction block to see if it clicks when you turn the ignition switch to start. I would try swapping it with another relay. I’d then suggest scanning the computer for input from the transmission neutral switch to the computer if the problem continues.

Since you can’t get the car started, consider getting some assistance from a mobile mechanic such as YourMechanic. A qualified technician can come to the location of the car to diagnose your starting issue and fix it accordingly. Good luck.