When I first read this, I thought pinion angle was off. But Steve M's solution seems pretty sane.
Try this test. When you pull off the highway, as you're barreling down the offramp, push the clutch in or shift into neutral. If you suddenly hear the noise, it could be pinion angle. Otherwise, it probably isn't.
I say this because I have a '94 Ranger with a slightly-off pinion angle (I believe). It only makes noise under very light throttle at highway speeds, and under the conditions I described above.
At the same time, Steve M's also right about the little noises slowly driving you into an asylum!