Rear Axle Spindle problems of the 1999 Cadillac Deville

I have a 1999 Cadillac Deville, there is a problem with the rear suspension knuckle assembly, I have had this car to 2 different dealers, the second dealer, last summer, replaced the rear struts & stabilizer bar which did not rectify the situation when ambient temperatures are above 60 degrees, I hear a "fierce" screech/scrunch sound from the rear suspension, I have gone round & round with Cadillac customer service & the dealers, now that the car is no longer under warranty Cadillac doesn't want anything to do with it. I had my oil changed & the tires rotated last week & while they were under the car I had them to look at the rear suspension, they said it needed a new knuckle assembly & the dealer knew better than replace rear struts with this type of noise. It seems that nourse Cadillac in delaware ran up a big tab & billed it to Cadillac (warranty) and as soon as they made their money they wanted nothing more to do with the car so now I am stuck. A car of this magnitude & price should not have rear components wearing like this, I only have 77,000 miles on the car & I do not drive it hard. . . . What do I do?.