If it's an 8.5 with 2.73's it's a relatively easy swap. If it was 2.56 or lower (numerically) it would require a different carrier (or a spacer). 3.42 is the factory gear in '84-87 Buick Grand Nationals. I'd expect to pay around $200 + parts if done under the car; maybe a little less if the rear is out and can be done on the bench. Find someone with the equipment and knowledge to do it right the first time.