nthinuf

Don't go to the cheap place just because of the price.

Be sure to check the warranty. (how will they handle any issues?)
Ask how many JK's they have done. (JK's use thicker gears than previous jeeps, so the setup may be different than they are used to)
Try to find reviews online, or negative reports with the BBB.
Get referrals from local 4x4 clubs. (this is a great way to find a good shop!)

JK D30's with stock 3.21 gears are the only ones that need new carriers. As noted above, the carrier is less than $100, and if you do not have 3.21's, you do not need it.

Just a heads up to do a little homework to hopefully help keep yourself from getting screwed.