Do you remember what the backlash was in the gear set when setting up the spacers? With an indicator, you should have had about .0100" of backlash between the ring and pinion. May be to tight..
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2000 V6 .030 over comp cam, k&n cai,
underdrive pulley set. mac headers, mac h pipe flowmaster catback, aluminum ford racing drive shaft ford racing 4.10 posi. And a 05 V6 for every day use.
Using shims of different sizes, they move the carrier with the ring gear to the right (tighter) or to the left (looser) to make up the space between the gear set. With an indicator on a tooth of the ring, it should rotate approx .0100" before the pinion starts to move.
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2000 V6 .030 over comp cam, k&n cai,
underdrive pulley set. mac headers, mac h pipe flowmaster catback, aluminum ford racing drive shaft ford racing 4.10 posi. And a 05 V6 for every day use.
Its happened to me before, thought i'd throw in some advice. If you've alleviated that it is not the rear end or a gear issue, then what i did is i checked the rear brakes. If you have rear disc brakes and not drums sometimes you can clamp the caliper onto the rotor a little to tightly, causing the pads to rub when they heat up and cool down quickly. I.E. before you get on the highway you stop at stop lights warming them up then get on the highway and they cool down and expand. Although thats if they're riding the rotor. Just a helpful hint and maybe one more alleviation to your issue. Hope it helps.
that also could be it since i did the breaks also yesterday, but it does not sound like it draging cause when i let off the gas it stops, but as soon as i get back on the gas it starts to humm again.
The humming noise 
