freebsd vpn server behind nat dsl router

[Robert Johannes]

Hello Greg,
I am writing you, because I saw your responses to a couple of messages on 
the freebsd-security mailing list related to freebsd vpn and nat.

My situations is rather unique, and I am needing an expert's eyes to 
glance at it and confirm whether it is doable or not.  I have a simple 
diagram that illustrates what I am trying to do, and it is located here 
(about 40k): http://www.hamline.edu/~rjohanne/lan.jpg

In the diag, the dsl modems have dynamic public ips on the internet side, 
and private ips on the lan side.

As you can see in the diag, I am trying to have the vpn traffic from the 
internet forwarded to the Freebsd vpn (the machines ending in .254 on each 
site).  I have followed the Freebsd "VPN over Ipsec" in the handbook, and 
created a tunnel between the two vpn servers; according to the handbook, I 
should be able to ping the vpn servers using their private network 
addresses, but I am not able to do that.  I realize that my implementation 
is not exactly like the handbook's, but what do I need to do to get it to 
work?  I have googled, and researched all over the net without much 
progress.

I have seen a lot of messages related to nat and enabling vpn passthrough 
on different dsl modems and so forth, which I have tried to do, but still, 
no progress.

Any clues and pointers would be appreciated.

thanks
robert