Truck just clicks when you try to start it, and it is not the battery.

Have you check the starter and solenoid

The clicking sound that you hear is most likely the starter solenoid,,so there is a chance that either the solenoid is bad or the starter itself is bad --- replace.
**** now I know that you said that " it is not the battery ', but most of the time when the solenoid is just clicking like that, it is a sign that there is not enough voltage / amps to engage the starter,, just to totally sign off on the battery -- try and jump start the car using another car or a fully charged jumper pack,, if it still does not start then it is the solenoid or starter ( which are sold as a complete unit together ) -- replace it and all should be back to normal.

c...

Its the battery.