@Aethrios:
Now, the PVID settings I have on the switch give me some pause: I can only set one PVID per port, even if the port is used in more than one VLAN. Clearly, port 3 (pfSense) is a member of both the WAN and LAN VLANS, so I'm under the impression that it needs to be configured with a PVID of both 2 and 3, in order to properly pass frames between the different ports. Also, I've tried setting the untagged modem port ( 8 ) to a PVID of 2, since it's on VLAN_2. I believe this is correct?
The PVID setting simply controls what the port does with untagged traffic that passes into the port (traffic coming into the switch from a connected device). If you want untagged traffic on a port in say, VLAN 50 (my VLAN number is arbitrary) you set the PVID to 50 and set the port untagged in VLAN 50. That would handle a client device that is unaware of VLANs. The PVID makes sure any traffic entering the port that doesn't already have a VLAN tag gets tagged as 50, and the untagged setting in VLAN 50 makes sure that any VLAN 50 traffic passing through the switch will exit the port, removing the tag in the process. You can only have one PVID per port.
Hope that makes sense.