Either pull the torque tube ball back and inspect the U joint, or drop the rear end cover plate and see if the ring gear spins. I'm going to go with the broken axle option over the U joint, as there would probably be some rattling noises from a broken universal.

With a broken axle, the ring and pinion will spin, one axle will keep stationary, and the spider gears will rotate around the stopped axle.
Jerry


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