Well, I am certainly not an expert on these trucks (by a longshot). First, the service manual I have (for a 2013 Frontier) shows 700 rpm base idle +/- 50 rpm for the automatic in N or P. That means the range of acceptable base idle speeds is 650-750 rpm. It would make sense (logical) that the AC would make the problem worse, because of the additional load on the engine while it is already idling too low (630 rpm) without the AC on. I'd take the path of least resistance and find an independent repair facility specializing in Nissan and ask them to turn the idle up to 700-750 rpm. Do you have the service manual for your truck? It is available online...Also, I have an OBD reader permanently installed with an app on the wife's iPad...might help you in diagnosing and is very inexpensive (less than $30 reader and app).