If the shop manual covers more than just the engine, etc., it will probably have a section on drivetrain components. (Transmissions, rear axles, etc.) That's the one you'll need to find for backlash and preload specifications. I'm not suggesting your guy doesn't know what he's doing, but rear axle repair is a very specialized skill that only a few professional mechanics bother to learn. The tooth contact pattern is lapped into place when a gearset is manufactured, and even a slight mismatch on assembly will result in a gear howl or a whine that's almost impossible to correct. As a good rule of thumb, new gears need a backlash of around .002" to .008", and pinion bearing preload for new bearings should have a turning torque of about 12 to 20 inch-pounds. That's set by tightening the pinion nut with a new crush sleeve between the bearings until a slight drag is created when turning the pinion gear. It usually takes 200-300 ft-lbs. of torque on the pinion nut before the crush sleeve starts to compress.
Jerry
