Well...Cox are as the name suggests. They're not p2p friendly. I'd take a wild guess & say that your problem is there.
If you use the forum search to look for Cox, you'll find heaps of threads where people have had trouble (mostly with connection & downloads though). Search for no uploads Cox & you'll see you're not the only one who's had this happen.

Since you're not firewalled & seem to have done everything right, I'd say they're throttling your uploads.
You could call them if you like & ask why but to be honest I don't know if you'd get a straight answer.
Sorry I can't help you more. At least you can download & you're trying to share...it's not your fault if your ISP are stopping that