I would most definitely do FJ springs front and rear, they improve the ride and it just feels awkward having the front only (I have only fronts, can not do the rears with my suspension). It all depends on what look you're going for. If you just want level, do the springs only and it'll be leveled out. If you were a v8 or 4x4, you'd need to add a small spacer to get it leveled, but my guess is with the v6 2wd you'll get enough from just the FJ springs. People run FJ springs with the 2.5/1.5 kit, but the FJ springs don't provide as much lift on their rigs as it will on yours. v6 2wd is much lighter than the 4x4's and v8's, so that's why you'll gain more lift from the front springs than others have.