I just installed my Nitto Trail Grapplers. 285/65r18s These are my first set of aggressive tires. I notice that at low speeds <20 mph, there is a noticable vibration. Is this due to the big tread blocks or something else? I ordered them and had them mounted and balanced by 4wheelparts. The shop said it is probably a bad balancing job or a bad tire. Any ideas? Could it just be the tread pattern? Help please and thanks.