Why is the right side of my 2004 Nissan Xterra.

Adjust the torsion bars.

Every tire and alignment shop will have a small book that lists every vehicle, and where to measure and what those measurements should be. You can't do anything about the rear if it's low except to have the leaf springs worked on, but more commonly the front torsion bars sag with age, and it's pretty easy to adjust them. That is supposed to also be a part of every alignment.