Well to be honest I would not think it is either of those 3 if it went away after unplugging the t-case. My mom's '00 likes to try and lock 4wd at speed, making this awful grinding sound. We pulled the fuse under the hood and it drives fine now. Basically deactivates the magnetic clutch that operates the A4WD in the t-case.
If you had unplugged it and it was still there, then I would say it is a suspension component since they are moving regardless. That being said, if the hubs haven't been replaced, it wouldn't hurt to do them. Mine went out around 160k. They are pretty easy to replace.
https://www.youtube.com/watch?v=OCr8XyTEGmQ
Same method as on the expy. One thing I will warn against is that when I did mine, I could not get the rotor to separate from the hub, so I just replaced those too while I was in there.