Well an open differential works like that with all the torque going to the wheel that can spin. Same thing happens when you are in a slippery situation, if one wheel can slip/spin the other wont... So I am assuming you are not on a slippery surface and one rear wheel is dragging across the ground/not moving and the other is turning/trying to push the tractor across the ground(in a circle)? As you guessed, that sounds like a stuck brake, or something is seriously broken inside where the rear differential and axles meet. Does it do it forward and reverse? Does the stuck wheel move at all when you switch from forward to reverse. Any movement when going from forward to reverse might indicate a gear problem. if it is frozen in place/no movement in either direction, I would say it is a stuck brake.
Where the brake linkages enter the axle housing is where the brakes are located. On the bottom of the brake housings are drain plugs. Remove them and see what comes out. you are hopeing they are dry and not full of water, but tractors who live out in the rain can sometimes fill up with water. Has also been known to happen in the clutch housing and it fuses the main and PTO clutch discs(also has a small drain plug)...
Do the brake pedals move? The Jinmas have a brake lock of sorts, it is a metal tab with notches on it that engage a pin on the brake pedal. Is that brake lock disengaged? If you follow the brake pedal linkages from the pedals back under to where they enter the rear axle housing, if the pedals are moving are the linkages also moving all the way into the housing?
These tractors also have split brakes, with a little metal tab that connects the two pedals together. These are to help you turn in sloppy conditions, or to apply brake force to a wheel that is spinning when you are stuck in the mud to send some torque to the wheel that has some traction and is not spinning, to help move the tractor(first generation traction control
). This would be the first thing I would try if the brake linkages are moving. flip that little plate out of the way so the brake pedals operate independently, Start the tractor and in low range, first gear, step on the pedal for the wheel that is spinning, which will force some torque thru the differential to the wheel that is stuck. Try this forward and reverse and see if it gets something moving...
These tractors also have a rear differential lock(lever is behind right leg) but I would not use it for this as the mechanism that locks the axles together is a little fragile. I only use it when I am stuck and wheels are in a condition where they can definitely rotate and or slip...
