If you can get hold of an old Ethernet hub (not a switch) all traffic appears on all ports. They're a bit hard to find now and only operate in half duplex. I used to keep an old 100Mb hub at work for troubleshooting with Ethereal (Wireshark). Now days any half decent managed Ethernet switch will let you mirror traffic to another port.
I doubt trying to physically tap the wire pair would even work, if it did you're only going to see half the conversation, i.e. The Tx data to Rx data in one direction. A full featured switch port mirror will mirror the tx and rx traffic on one port out of the mirror port so you see the whole 'conversation' in the Wireshark capture.
The alternative if you can do it, is to run Wireshark on one of the end devices, then it sees the tx\rx traffic and you don't need any taps or mirrors.