Policy Routing / NAT Question

[Ryan Wilkins]

Hello..

I'm setting up a TCP protocol testbed for testing various enhanced TCP
stacks for use over high bandwidth, high latency links such as
Satellite.  Due to hardware limitations of the FreeBSD boxes we're using
(1U rackmount), there are no expansion slots left for additional network
cards.  This forces me to have to run some traffic over one gateway and
the rest of the traffic over another gateway, all through one ethernet. 
The problem I have is I need to change the source address of some
packets based on destination address.  I was able to accomplish this in
Linux using Source NAT.  I'm not real well versed in FreeBSD hence the
reason I'm asking for any assistance here.  Does anyone have an idea how
to change the source address based on destination address?

Thanks in advance for any help.

Ryan Wilkins