Hello the first I'd like to say thank you for taking the time to read this. About two years ago I took my 2005 Ford Expedition to the mechanic for a rear differential repair. Even though it was under a third party warranty the mechanic billed me $400. This was a Ford dealership. The mechanic told me that it needed shims as well. But after being nickeled and dimed over what I thought was under warranty I decided to take my car home anyway. I'd like to try to repair this issue myself. My rudimentary understanding is that shims would go around an axle preventing the wheels from wobbling. Basically filling in the Gap that may have formed due to the differential replacement or some other wear and tear. Can someone please explain to me what this forward and backwards wobbling is when I brake going downhill and how I should go about repairing it. I am reasonably good with mechanics but do not quite understand what I'd be getting myself into on this one. I have pulled Transmissions off of cars and replaced clutch plates, bearings, alternators, belts, brakes, oil, sensors, but have never been familiarized with drivetrain axles and suspension. Thank you for taking the time to read and hope someone can give me advice on how to do this work myself.