Sorry to revive this deadthread, but I haz a question!
When I'm driving along at about 35-40mph, I hear an intermittent, but steady rhythm'd vibration/low noise, similar to the sound a tire would make at such speed when making slight contact with a fenderliner or some such soft/flexible material. It may or may not be a groan/growl, though it could be, since we all tend to describe our onomatopoeias differently.
I'm concerned that it's my center diff, though I have not ruled out wheel bearings, etc. It's not speed-sensitive, other than about 30-50mph on smooth, particularly blacktop roads. Heck, it could even be my tires, I can't say for sure. It doesn't seem to be noticeable on uneven or composite roads.
I've heard that they make noise when they're on their way out, but, is there any other definitive way to confirm that I have a center diff problem?