[blykins]

If it's off and on like that....My guess is that your backlash/pinion depth is off a little bit.

I'd yank the diff cover off, paint the gears and see what the shape comes out to be. If you have a mag base and dial indicator, you can check the backlash as well.

There's a possibility that it could be a bearing going bad, but chances are the gear mesh is the culprit here.