My 1994 LC has a strange noise at highway speed only when my foot is on the accellerator. The noise sounds more like a faint hum, and stops once the vehicle is coasting. My first thought was dry u-joints, but greasing the zirks did not cure the noise. It's a recent purchase and I haven't had a chance to change the transfer/diffs to fresh fluid and I'm unsure of the fluid last change. Any thoughts?