Apple's airport uses a NAT firewall. I'm surprised people could upload from you before this!

See THIS about Airport opening port & choose Port 6346. I'm not sure whether you will also need a static ip or not. I'm also not sure if Airport uses UPnP or not. If you still have problems then manually force ip within LW: Force IP instructions & sample image

Oh I misread your post. So port 6346 is already enabled thru Airport?!

Then make a note of all LW's settings & close it. Then delete LW's preferences folder found here:
/Users/your_username/Library/Preferences/LimeWire