CV joint

You have three different problems. CV joints don't rattle unless they're ready to fall apart. That's pretty rare. Rattles are usually caused by things like anti-sway bar links, shock absorber bushings, worn ball joints, bad wheel bearings, and loose brake pads. A torn cv joint boot lets the grease out and dirt and water in. That chews up the six steel balls that run in grooves with very tight tolerances. You will hear rapid clicking when you turn and move forward or back up. That's the first sign the joint is ready for replacement but it can take months or years for it to actually break. The warning light has nothing to do with either of the first two problems unless the part that is rattling is the same part that's causing the light to turn on. You might consider getting a second opinion. If I'm proven wrong about the cv joint and it really is rattling, then you only have two problems. That's better than three problems.

I can only share that cv joints on Cadillacs were REAL expensive, ... In the 1970s when they were new to all of us. They are still very expensive to buy new ones from the dealer. A more economical approach is to buy remanufactured half shafts. That includes the inner and outer joints, connecting shaft, and two new boots. Your old shaft will be returned to be rebuilt and sold to the next person. Some of these rebuilt shafts can be found for as low as $80.00 for Chrysler products, and a little more for imports. Defective joints, such as yours, are only replaced when necessary but they will all have new boots. Some people replace the entire shaft assembly when just one boot is torn. Boot replacement kits cost around $20.00, and about twice that from the dealer, but it takes about an hour to replace it. The shaft assembly can often be replaced in less than a half hour so labor cost is less. I can't say about your car, but the big heavy Cadillacs in the '70s were real hard to get apart to replace the shafts. Those could take up to three hours and lots of frustration. I suspect your car will have one of GM's more common designs now and will likely not take that long to repair.