First, try disconnecting the door with the release rope to confirm the problem is not in the door itself. If the lift trolley still will not move normally with the door off then there is failure between the trolley and motor drive assemblies.
If you look at the parts diagram exploded view on page 37 of your owner's manual or on the Sears Parts site you will see the gear and sprocket assembly that turns to move your door up and down. That assembly is identified by the reference key number 2. Key number 3 is a worm gear that attaches to the motor and turns the gear and sprocket assembly.
Since your motor still runs but the door does not move the problem is probably a failure in this area. the 41c4220a kit should include all the replacement parts you might need to repair an issue there but it does require disassembly of much of the overhead opener and re-timing of the door trolley and limit switches, so if you have any reservations you may want to consult a professional.
The links below have more information that might help, some of the specifics apply to different models but the basics will be the same as on your model. You can price parts on the Sears Parts site to get an idea of some of the cost you might be facing. You can also order parts direct from that site or use the numbers there to get parts through your Sears Store.