1996 Chevy 1500 4X4 103000

I have recently changed my rear differential fluid and cleaned it up. I replaced it and filled it also added the additive fluid for lockers. The noise I was hearing before that is what I thought the problem was. I have a rubbing or faint grinding noise when I hit about 20-25 miles per hour. At first it only did it till I drove for a little bit then it went away. Now it is constant. I looked at all my gears and no teeth were missing and there didnt seem to be much play. Could I have like a bearing problem on one of my axles. And can I check them? I havn't fooled around with rearends much so this is all new to me.