I have a AM Northern and luckily never had to do much to it. The previous poster has given a lot of good tips to resolve the problem. All I would say is that if you remove the side rods and take out all three axles with the intermediate spur shaft/gears and set them up so that they all align correctly on a flat surface and then drop the chassis on top, then you ensure that the quartering is in alignment for all three axles.
I can’t see why this is difficult, I assume AM have a tool that sets the axles in place for assembly, a five minute job at best.