How can I tell if my differentials need replaced?

Hello, No way. That will not hurt the differential. I always like to life cars in the center, but unibody cars do not always offer that option, and I do not jack up cars by the sway bar like I've seen some people do. To tell if differentials are bad requires quite a bit of advanced knowledge of driveline components, so your best bet at identifying if you can trust the service order is to get a second opinion. Look here for certified shops in your area that you can trust: http://repairpal.com/nissan-within-10-miles-of-salt-lake-city-utah-84119