This is going to sound dumb, but i still have a slight vibration/humming sound after replacing u-joints on my front drive-shaft. Question is, in two wheel drive is the front drive shaft supposed to be rotating? On my old s10 zr2 there was a vacuum actuator that engaged the front diff. So i assumed that the front drive-shaft did not rotate. But on the Titan the front drive-shaft rotates and i was thinking that maybe that is where the vibration/humming sound was coming from. I am stumped.