photogravity

IANAM by any means, but I do quite a bit of wrenching on my bikes. Based on your description, it sounds to me like the wheels may be a bit out of true. If you eye them up and they don't appear to have any visual wobble and if you are mechanically proficient, I'd suggest swapping in new pads for the brakes. As far as converting from v-brakes to cantis, I would not suggest making that change. V-brakes are simple, easy to work on and have incredibly good stopping power, even for a tandem.

Finally, most any good bike shop should be able to work on a tandem, but if you don't mind taking it to a specialist or don't believe your LBS is proficient, then perhaps a trip down the road may be worth it. I disassemble, reassemble and repair my bikes and only take one to the shop when I can't quite get something set up properly or don't have a special tool that I don't feel is worth investing in to use only one time, so I may not be the best person to respond to this specific question.