[shorod]

Sounds a bit like the standard rear-end noise that probably 85% of the Explorer owners notice and comment on. Some people have tried a different weight of rear gear oil with marginal success, some people have had the rear ends rebuilt, again with marginal success. Most just live with it.

You should consider changing out the rear gear oil and using the proper additive if your differential is a limited slip. Make sure to refill to the proper level. See if that changes the noise. Hopefully it won't make the noise apparent at lower speeds. While under the truck changing the oil, inspect the universal joints to make sure they don't show signs of having leaked out the grease.

-Rod