1999 Grand Caravan AWD rear axle noise
 

Just bought a GC with 117K miles on it. Runs great, except I'm getting a nasty noise from the rear drive axle (i think) under certain conditions. I'm looking for advice on what to check, and if anyone has ideas on what the problem might be (or how bad it might be).

I first heard the noise when driving in some heavy traffic in a rainstorm. It was stop and go, and I spent more than an hour driving less than 20 mph. In this range, I started hearing a whining noise from the rear. It got progressively louder, and eventually the whining was accompanied by severe grinding and clunking. To me, it sounded as though a gear was slipping. The noise would stop completely when the car was stopped, and resume again as soon as I began moving. The noise subsided when I could get my speed up, and there seems to be no problem above 30 mph. At low speeds it was bad enough that I considered having it towed, but fortunately I got out of traffic in time and was able to drive at normal highway speeds. I've since driven it several hundred highway miles with no problems, but I'm still concerned about the grinding that occurs in the rain at prolonged low speeds.

I put the car on a lift with a mechanic to check it out, and found a minor fluid leak where the passenger side half shaft connects to the differential. He topped it off, however, there was still plenty of fluid in it so I'm not sure that was the problem...

Any thoughts or ideas? Please share!