You do not need to replace the trees yet. If they are blooming, then they still have life and leaf drop on magnolias is caused be several things. Lack of water (unusual in the spring) or a blast of cold wind will cause leaf drop as well as root damage from an underground animal. Are the leaf droppers  more exposed to wind than the others? Do you have rodents underground? I suggest you add a mulch on top of the soil to keep it cool and moist such as wood chips or bark chips. Then wait to see if after blooming the leafless magnolias decide to put on new foliage. Sometimes it is just the shock of the transplant that causes leaf drop and because the two trees are newly planted I suspect they will pull out of this and you will be pleasantly surprised with new leaf growth this summer. Keep growing, Marianne Binetti