I have the same problem on my JD 4310. I got the brakes unstuck by getting under the tractor and tapping the levers that the brake rods attach to. There is a rod that goes from the front where your brake pedal is to the rear wheels where the brakes are. Both the brakes on my rear wheels were stuck. I had my son press the brakes and I could see the rods moving, but the levers they were attached to did not move. I tapped them with a hammer back into the position that disengaged the brakes and the tractor now moves as it is supposed to.
Problem is that if I step on the brakes again they get stuck. I talked to a service manager at the JD dealership and he told me to spray a penetrant (liquid wrench) into the mechanism and just work them back and forth until everything works again. I still have to do that. I also need to replace the springs under there since they appear to be rusted and not functioning. They don't have enough strength to pull the brake levers back to the disengaged position when I let off the pedal.