I always figured the easiest way to drop the back would be to pick up an Xterra rear end. It is already spring under rather than spring over like the frontier. I'm not sure of the exact drop amount, but it may be as much as 6". You may even need lift shackles to bring it back up a little. Shorter shocks and coilovers could be all that's needed otherwise. Maybe adjustable upper control arms to dial in the alignment also.