Hey Bud, I hate to tell you this but I was experiencing the exact same thing back in February after driving on a pretty bad gravel road. My first thought was wheel hub or bearing. I drove it with the noise for about a week then the Tranny went out and it wouldn't shift out of 1st and had no reverse! 2 weeks later and out $2,500 I was back in the driver seat. But I guess $2,500 is better than having a monthly note... Good Luck.
I'm hoping it isn't something like this, although I have been known to have terrible luck. The Transmission has never even slipped a gear, hesitated, or shuddered. I'm hoping its some bearing or something that is exposed to the outside.
2 days in the dry, and the sound is non-existent again. So I'm thinking Hubs might be the culprit. Gonna have to check for play, and get under neath it.