So basically what you're saying is that the game is too much like real life? Because if I was equally proficient with firearms and bladed weapons in real life, and I could use both my hands equally well, there should be nothing keeping me from putting a gun in one hand and a long knife in the other, right?
What exactly is the problem with being able to use all of your abilities in any class? It's not like picking up a sword makes you dumb as a box of rocks and unable to cast magic. If you want to tank, choose a weapon and tank. Just tell your party members.
The MOST ANNOYING THING about ALMOST ALL popular games is the COMPLETE AND UTTER LACK of gaming ettiquette. A lot of people just walk up to you, invite you to party, and when you ask them why THEY WON'T TELL YOU. I literally have found that most people will not take the time to type something out even if they're asking you to trade. They just open a trade window without saying a word to you. It's so bad that it seems that most people don't even realize that you can talk to each other in these games.
If you want to tank, just say "I'm tanking." If the other player doesn't know what tanking is, then it really doesn't matter. Wearing a shield isn't going to make them any smarter. As a matter of fact, it would probably be better if you DIDN'T play with this person, since the fact that they don't know what a tank is probably means you're going to die a lot.