If the balance and weight on the garage door test okay, then the most likely issue is something binding or jammed in the gear and chain/lift assembly itself or a failure in the worm gear/drive sprocket area. There is more information in the links below that should help you pin-point the problem. Some of it deals with other models but the same procedures should apply to your model as well.
The model number you supplied looks incorrect but is probably a 139.53673SRT rather than a 5RT, since it is easy to mix those two up and it happens fairly commonly. If so, then you can also use the information in the links and that number to find your manual at Manage My Home for more detailed and specific information or parts online through the Sears Parts site.