Agreed, most likely, the SQL Browser port. you can test if it is open by start a command prompt window and typing telnet servername 1434

you can also check if it is trying to use SQL Browser by running folling in command prompt window whil trying to connect with your linked server

netstat -an | find "1434"

If there is no communication between the you on port 1434 it will not return results on that port. If there is a row with a status of SYN_SENT then it will be networking (firewall/router ACL) that is the issue.