Your tractor acts like the worm is not turning. Your idea of a stuck brake has merit. but maybe something broke and got jammed inside somewhere.
The "proper" way to move one of these tractors is to remove the wormgear out the bottom back bearing bore. Jack one wheel off the ground to take any load off the worm and bearings while you do this. Working that wheel back and forth might help squirt the worm out of there. There"re gallons of oil that will run out on the ground. Catch it and poor it back in if you intend to move it more than a few yards.