Frontiers have a sheetmetal strip under the grill between the bumper and the grill. It extends around, and the fender extension under the turn signals are a different length between Xterra and Frontier fenders.
I think the Xterra grill substitutes more plastic in where the sheetmetal strip is on the Frontier, but I still think you'd end up with a good gap between the bumper and the grille.