I have a 97 Grand Cherokee, AWD 5.2L 318. between about 25-60 mph I can hear this strange humming noise coming from the mid-rear end. to put it into perspective, it sounds similar to a bad wheel bearing, which is what I initially thought. but it only makes the noise when accelerating. It also vibrates slightly when put into gear. U joints and motor mounts appear to be tight and in tact...Im stumped, please help