Told by dealer that rear differential bushing is worn and needs to be replaced.

My first question is what prompted anyone to inspect the bushings. That is not something that would typically be evident when they do a quick visual inspection while the car is having other work done.

I would recommend getting a second opinion from an independent shop. Normally I say to not tell them what another shop found. Let the second mechanic do his own inspection untainted by what others have found, and see if he comes up with the same thing. In this case, however, the bushings could legitimately be worn and need to be replaced, but that is not common and almost everyone would overlook them during their inspection unless you specified you were having a problem or concern that would prompt them to look for things like that. You may need to ask to have the bushings inspected at the second shop, THEN let them give you their estimate for repairs.

All the bushings are the same age so if one needs to be replaced it is customary to replace all of them. Most shops use a "flat rate" guide that lists the hours allowed to do every procedure so they all charge you the same amount of time. The only variables are the hourly shop labor rates and any small additional stuff that some shops include in the jobs and some charge extra for.