In FreeBSD 5.4/i386 (and older) dhclient have an option '-r' which is now missing.
No equalient is found.

From 5.4 dhclient manpage:
       The -r flag explicitly releases the current lease, and once  the  lease
       has been released, the client exits.

Also the script functionality in '/etc/dhclient-enter-hooks' is incompatible due
that old resolver config update function used the name make_resolv_conf(), while
the new one use add_new_resolv_conf(). End result being that old replacement
hooks won't work. I use a predefined resolver config.

Fix: 

kill -9 <pid..> will partially do the same as the option '-r'.

Below /etc/dhclient-enter-hooks will workaround incompability bugs with resolver:
make_resolv_conf() {
  /bin/cp -pf /config/resolv.conf.luth /etc/resolv.conf
  }

add_new_resolv_conf() {
  make_resolv_conf
  }

Provided that admin is aware of it..