Loud howl above 70 mph, but only when on the gas

The sound seems to be coming from right under the middle of the truck. It starts at 70 mph and continues to 80, which is the fastest I've tried. In that 70-80 mph range, the howl only appears when I'm on the gas. I don't need to be accelerating; just cruising at a constant speed induces the howl. If I let off the gas, the howl disappears.

When I downshift from 5th to 4th at a constant speed, the howl doesn't change at all. Same pitch and volume. That tells me that it's something after the transmission (u-joints, spicer bearing, differential). Is that a correct conclusion? What are my next steps toward diagnosis?

Truck is 2005 DC 2wd with 227k miles. I've had this truck for a year and around 3k miles, and I don't have any service history for it.

 

If you can get it on video and post it here. That be very helpful.

 

Sounds like rear end/gears. Check your rear diff fluid level and for leaks.