Bad rear wheel bearings will get usually get noisey before they have real problems. To isolate the noise, you can load/unload a particular wheel bearing by gently swaying the car side to side while driving. The bad bearing will get louder when it's under load and quieter when unloaded. In a non-Chrysler situation I had with one of my cars, the bearing surface on the axle shaft was deteriorating and the bearing ended up wearing into that surface. In that case, the "bearing noise" went away when I applied the brakes and the drum brake shoes centered the axle in the bearing and the noise went away momentarily.
Other than u-joints or a driveline vibration, the other "popular" fix these days is "road force variation" in the tires. We used to call it "hard spots" where the tire would balance (usually bubble balance, or if you wanted a real deal, the high speed on-the-car spin balance) fine and still have a vibration at speed. It takes a special balancer to read road force variation, but it works similar to the other computer balancers where it tells you where to move the tire on the wheel and retest for road force and balance.
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