Not to poke at you, but all that should have been fairly easily spotted before buying.
And why do you need a whole new rear diff? That’s ridiculous.
Dodge is no longer producing many parts. Flat out discontinuing them, even on cars they are currently producing. It is faster, easier, and more profitable to R&R a rear end than take one apart, fix it, reinstall it. Not to mention they no longer need specialists that have to be trained and available at eveyr service location. They just need parts swappers.
To be fair, Nissan and many other manufacturers are doing the same to cut service costs/time and increase profitability. That will become the new model for all of them in time.
That said, dude, find another shop. You are being robbed. 1600 for brakes? 2000 for a front diff seal? Robbed. Stop going to the dealer. Go to a brake shop for brakes or do it yourself. U-Tube is your friend. Go to a transmission shop for the diff work. They do it all day long. And dont tell them how much you paid for what already. They will be signing up their kid to Stanford when you walk out of the door.
As far as parts, do not buy them new. I promise you that you can find a rear axil at a salvage yard for 1/3rd of what you are paying. You could have gotten a whole front diff for what youu paid to fix a seal. You could almost buy a set of Brembo brakes, new rotors and pads for what you paid for the brake job.
And next time you buy a vehicle, have it checked out by a mechanic. Clearly you missed a lot. Even if you pay one a hundred bucks to check it out, you would be way ahead. Also do not buy a vehicle with ANY mods on them. Rims, CAI, anything. It is a sure sign someone treated it like a toy and if they did, they probably drove it like a toy. If you want mods, find a car that is unmolested and do it yourself. Cars that are unmolested are more likely to have been treated as transportation and not a play thing.