It is extremely hard for a dealership to prove abuse on a car. Somethings are obvious, and some customers admit it ("I missed a few shifts"), but if there is no actual proof (a technician can't just say "I've never seen that so bad"/"never seen it before") and make it stick. If your car is on the internet crushing cone courses, you're in trouble. If it's just normal use, then keep reading.
If you're under warranty, get with the service manager, service director, or "fixed operations manager" if they're playing games with you. If they don't budge, you have the right to get in touch with Cadillac's zone (or district) manager. Don't let them show you the door, you paid to have it inspected.
Remind GM that you'd really like to buy a new Cadillac one day, and that customer service goes a long way in this industry. If you're under warranty, and this is turning into what we call a "known issue", make sure you get what you want. Do not threaten lawsuits or boycotts, or any crap like that; do you really think dealer principals can't afford good attorneys? If you don't get anywhere, you have the right to arbitration.
"Owner participation" (that's where the manufacturer tries to get you to pay for some of it) is not an issue if it is not:
1) Wear item, clutches, tires, brakes.
2) Caused from lack of service/routine maintenance (have you had/required diff fluid changes?)
3) Still under basic warranty or powertrain warranty. Differentials are powertrain, although I'm not aware what the '05 powertrain warranty is on GM cars.
If you're in warranty, did all the maintenance to a tee, and they cannot prove excessive use (or damage/unauthorized repair), then they should cover 100% of it. If you're even one mile or one day over the warranty period, the ball is in their court, or at least, plead for goodwill authorization/assitance based on the tremendous cost of a non-wear/non-maintenance item.
Dealers want a good score from the owners, and helping you helps them, and in turn, helps the brand. Cadillac wants young buyers and young owners, as they have been trying to shake an image that's as old as the brand, and thus, as old as the average customer. Cadillac is not a Chevy (no offense to Chevy owners), the same way a Lexus is not a Toyota, et cetera. Force the issue, and don't let them screw you over with excuses. That's the GM that got in trouble in the early-1980s with crap trannys and lousy customer service.
Good luck.
