If you get a GM W-Body (Chevrolet Monte Carlo/Lumina/Impala, Buick Century, Pontiac Grand Prix) in the shop with seized rear calipers it could be the case the slider pin has corroded and welded itself to the metal sleeve of the bushing. This happens when the lubricant burns off and water intrudes. This happens even faster when the plating wears away. This is why soaking the pin and sleeve in WD40 and cleaning only works for 10,000 miles before the caliper seizes again.
Cardone has done something about it. Their redesigned slider pin and dust boot holds more high temperature synthetic lubricant to prevent the pin from seizing.