That's a low speed for a tire/wheel vibration. You need to find somebody with a reed tachometer. A lot of high end dealerships will have one in the servier department. If you know someone there, they might let you borrow it while they ride with you. They are NOT cheap and I'm sure they wouldn't just chunk it at you and say "sure you can use it." It will tell you very quickly at what frequency your vibration is occuring. Low frequency vibrations point towards tire/wheel/axle and high frequency to a driveline vibration. Does it pulsate and drone and sound like a heard of pissed off bumble bees? That would be driveline. If it shakes the flooboard and seats and your butt, that would be tire/wheel/axle.