I don't know how to describe the noise, but when I let off the gas at higher speeds, I get a hum, or a slight buzz. Any idea what this is/could be?

 

Little more info would help...year/lift/tire/sye?

 

Most of that is in my signature...

89 cherokee
31x9.5r15
3.5" metal cloak

no SYE.

It does not happen below say 55mph, so only at freeway speeds.

I can extend it by letting off the gas a bit slower, if that makes sense.

 

On mobile so I can't see your Sig

Anyways, it sounds like one of your rear joints is failing, probably the one closest to the rear end. Just give em a look for play or loose bolts, when you lift it without an sye the drive line angles play havoc on universal joints, I've had it happen several times... But, at the same time if it's only a noise but you can't exactly feel the vibes, could be a hub, front u joint, or even just tire noise

 

Pinion movement in the rear diff?

Max

 

Replace rear U-joint first.

 

I know I have a front u joint that is bad (drivers side) I'm going to replace those first.