The condition in neurological psain is different and the mechanism of pain in neurological pain is also different. that is why you find some drug from different group which is prescibed for these type of conditions.
As far as the diclofenic is concern it is typical NSAID
and main mechanism of relieving pain is decrease the inflammation, so if this pain is because of some infalmmatory changes than it can help, but un cases of shingles
the pain is neurological and need different group of drugs for pain relieve and another thing is that you are using id topically where there is no cause of pain and to produce any effect it need to go to the site of pain.
I would suggest you to go to your doctor and have some medication which is effective for neurological pain.
Thanks and hope this will help.