As has been noted, the trucks have grease ports, so why do a major disassembly to lubricate the gear boxes???
When I got my SW-7 a year ago, it had the same problem--making a grinding sound. I amply lubricated both gear boxes through the grease ports, and the grinding sound was gone. Why do more?
BTW, I really love this switcher. The slow-speed operation is outstanding. I have so much fun making up and breaking down trains at prototypical speeds.