Have you tried this file:

/etc/rc.conf.d/dhclient

I'm guessing based on this:

[23:41 gw01 dvl /etc] % sudo grep -r background * | grep dhclient
grep: motd: No such file or directory
defaults/rc.conf:background_dhclient="NO" # Start dhcp client in the background.
defaults/rc.conf:#background_dhclient_em0="YES" # Start dhcp client on em0 in the background.
rc.d/dhclient: background_dhclient=$(get_if_var $ifn background_dhclient_IF $background_dhclient)
rc.d/dhclient: if checkyesno background_dhclient; then