Thank you all. Someone suggested "Harbor Freight", but I didn't really
see an electric nailer there that can do the job. Perhaps a specific
model number? Renting a pneumatic nailer is not really practical,
since my fence boards get knocked down periodically when baseball
season starts at my neighbors house. And Hamering is a problem because
this is a very old fence and using a hamer will just as likely knock
the fence down instead of repairing it. So many problems....

No problem at all. A couple of wraps of duct tape should tame any wayward
picket. And any of the wayward neighbours kids too. Make sure you don't
cover their faces completely, leave a gap so that they can breathe.