I am not a professional mechanic, but have driven these tractors for many years. As a guide if travelling on Tarmac in top gear at say half throttle, you should be able to stand hard on the brakes and lock the back wheels.....ie skid. If you can't they need attention. Chances are one side will lock the other wont, in which case they need balancing.
As mentioned above get a professional to sort them if you don't know what you are doing, new shoes aren't expensive, and fitting is straight forward..shouldn't take more than an hour per side.
Regards