There was a recent thread about another guys tractor that was doing pretty much the same thing as yours. The problem on his was all the bolts on the ring gear had sheared off. With the tractor jacked up and the four tires off the ground, all four would rotate when driven by the engine but he could take his hand and stop the front tires from turning. There was just enough friction from the ring gear to the carrier it (used to be) bolted to so that it could still transfer a little bit of torque to the axle and turn the tires. Once the tires encountered the least bit of resistance, they would stop turning.
Sounds like you may have a similar situation with yours. Good luck and hope you get it figured out.