Rear wheels locked up when driving 1924 TT. With axle in the air, both wheels turn in opposite directions but will not turn together. Axle is a Ruckstell and shift seems to move O.K. U-Joint??.
I would vote bad ujoint myself. If you unbolt the 4th main ball cap and look in the hole you might be able to see the problem.
Could be a broken worm, or thrust bearing allowing the worm to move and lock the driven gear. I've seen this occur in industrial worm gear reducers.