Thank you for choosing ManageMyLife.com. I understand how frustrating it can be when only half your agitator is working. The top half of the agitator is controled by agitator dogs. If it is not functioning correctly, your dogs are worn. I have included instructions on the images below on how to disassemble your agitator for dog replacement.
SAFETY NOTICE: Be sure to disconnect power to the appliance before performing any repairs. If you are not comfortable with doing any voltage checks that might be recommended, please contact your local service provider for repairs. Be aware that there could be sharp edges inside the appliance that could cause cuts or other injuries to yourself or others.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services
If you have further questions please let me know. Thanks for using ManageMyLife.com.