You can't redirect one port to multiple machines, your options are:
1) redirect different ports to different machines, i.e.:
rdr pass on $ext_if proto tcp from any to any port 1024:5000 -> 10.0.1.32
rdr pass on $ext_if proto tcp from any to any port 5001:65535 -> 10.0.1.33
2) get more external IP addresses.
/jkm