Unless you have an LJ . . . most of the time, anything much over 2" is going to cause vibes, which is why most 2.5" and 3" kits come with a TC drop kit. As mentioned a RE 3.5" is really a 4" kit, so to do it right, you should go with the SYE and CV DS . . . along with all the other stuff you need like a CV TB bracket, adjustible rear UCAs, etc.