If you are buying a kit, they are selling you some extra things you may or may not need and discounting it in the hopes you won't need it and they will have still sold it to you for a profit. If you buy just the springs and shocks(of your choice), you can do fine. As was said, you can talk to 100 Jeepers and you will hear as many stories of vibes and no vibes. As much has to do with the loading of the vehicle, the age and use of the U-joints prior to lift, and driving style. Its a crap shoot. Good thing is the T-case drop kits can be had for cheap if needed. You can lower it a little at a time with hardened washers and new Grade 8 bolts for little money. This will give you the ability to tailor it to make the vibes(if present) go away and not drop it anymore than you have too.
Kits are nice cause you can get things a little cheaper. Problem is they often include some things you don't need. Either way the springs from OME and some good shocks will be the answer to your load carrying problems. Stay away from spacers. You need more spring to carry the load. The spacers will lift the frame off the axles but the spring will still compress fully under the load, because they have a lower rate than the OME's.
I feel like I should have stock in ARB with all the OME marketing I am doing, but the truth IMHO is they are the obvious choice for this application.