Whistling/Squeal Sound From Back Wheel at Freeway Speeds

Hello,

I have a Toyota Tacoma (2x4) truck. Just recently, I noticed that
when I'm driving at freeway speeds (over about 50 mph), I can hear a
high pitched squeal (whistle?) sound coming from the right rear wheel
area.

The funny part is that I hear this most when I let up (or take my foot
off of) the gas. When I'm accelerating at high speeds, the sound goes
away. As soon as I back off the gas pedal, this squeal immediately
comes back.

Does anyone have any ideas on what this might be? An axle problem?
Wheel bearing? Tire?
Any advice or tips is greatly appreciated!

Thanks!
Brad