$650 doesnt really seem that cheap if you actually know what they are doing, and if you have cut all the mounts off yourself

as all they need to do is get one axle cut down and resplined. say $100-150
then they just cut a section out of each side of the diff adn weld it back together in a jig to keep it straight

obviously price will go up if they have less experience with it as they will be charging you for time to measure and figure out how to cut it down