natd2, fwd GW2 - not reaching destination correctly?

[G Bryant]

Hi all,
I have a problem i'm trying to figure out with FreeBSD5.4, natd and ipfw 
fwd command.
Have a box acting as a gateway with relevant kernel options compiled in.

External NIC with inet x.y.1.10 and inet x.y.2.10 alias
(I also tried using 2 seperate NIC's with the above 2 ip's, but no joy 
there either)
defaultrouter = x.y.1.1
I'm trying to send some of the packets out of a second dsl connection at 
x.y.2.1 using natd on 2nd interface and a fwd gw2.
Some reason the packets exit, but don't arrive at gw2.
I can ping x.y.2.1 from the console as I placed it in the same lan-space.

<-snip->
ipfw add divert natd2 ip from any to any out
ipfw add check-state
ipfw add fwd x.y.2.1 ip from x.y.2.10 to any out
<-snip->
e.g. ping 1.2.3.4 (from lan pc):
fwd command logged looks like this:
fw kernel: ipfw: 9200 Forward to x.y.2.1 ICMP:8.0 x.y.2.10 1.2.3.4 out 
via rl0

I have seen some older posts where people have specifically shown this 
as the way to do it,
but for me, the packets seem to have left their towel behind.

Any ideas?  Yes I know I've been told this isn't possible, but others 
have done it - so I just need to know what changed to stop this working 
(if anything), and what's the work-around / alternative now?

Thanks in advance
Gray