I've got a router with NAT and two machines hooked up that both want to play Star Wars on the zone. Unfortunately, my router does not support port duplication for it's forwarding. Does anyone know which ports Star Wars uses for internet play? And also, is there a way to force Star Wars to use a pre-determined port?

I don't know what port it uses, I'm also not sure that GB supports NAT, but to try and find the port why not try using something like wingate on your router(www.wingate.com) to see, that should enable you to see what port it's trying to communicate with, possibly.

SWGB itself uses DirectPlay - generally you can't change the ports it will want to use.
Anyway for forwarding you can use: 2300-2400 and 47624 - both TCP and UDP. NAT works fine if its all forwarded correctly.