I had a vibration between 1400-1800 on my 93 5.0. Balancer was wobbling all over the place, fan was cracked. Had both replaced along with a new fan clutch. Got a little smoother, but vibration was still there. Finally tracked it down to the flywheel. When the mechanic took it off the car and spun it in the shop, it was wobbly - as though it were warped or something. Replaced with a brand new one, and there was just a tiny resonance remaining afterwards, which has since either gone away or I don't even notice anymore. Harmonically, the engine runs and sounds practically new now.