As Axisg has pointed out, you should be careful if you decide to go for this. Have the seller pull the cover off, so you can inspect the gears, and make sure that nothing is broken. In the ad, its says that the axle is from an '87 Monte SS. If true, then it should have 3.73/1 gears in it, not the 3.42/1 gears. The 3.42 gears only came from the factory in 1983 and 1984. What year is your Calais? GM changed the wheel studs from SAE threads to metric threads starting in 1982, so if your Calais is an earlier year, you'll have 2 different types of wheel lug nuts if you go with the later rear axle-SAE on the front, and metric on the rear.
One other thing to keep in mind-while the higher numerical gears will wake up your cars performance, and will provide better acceleration, engine RPMs at highway speeds will be higher, and your speedometer will not be accurate. If that axle really is from an '87 SS, and it does have the 3.73/1 gears in it, then you might want to think about an overdrive transmission, because your engine will be revving in excess of 3,000RPM at highway speeds.
