Hi Paul, I put this one up on the bench to have a closer look, the reason they are heavy to push when not using the drive is because the rear wheels are having to rotate the gearbox which creates the resistance. Inside the drive gears somehow there is a sprag clutch that allows the rear wheels to rotate backwards when moving the mower backwards. If yours is locking up then there is something wrong inside the gear on one or both of them, I would assume it would only be one wheel and that is an easy test, sit the rear of it on a brick or block of wood and rotate the wheels backwards, you will then know which wheel it is. I haven't tried to remove the gear so not sure how it comes off.
