I have to second the spacer lift, they ride pretty bad most of the time.
A new kid on the block is the 6112 bilsteins, they are a 60mm shock so they are an upgrade to the 5100's. available for around $600 and use stock top mounts. they offer up to 1.5" of lift and can be run with up to a 1" lift spacer if you want a bit more lift.
You really don't want to run more than 2.5" of lift on a stock width xterra cause you will sacrifice ride quality.
For the rear, go for an add a leaf instead of shackles, yes the shackles are easier but the rear springs are all ready really soft and adding a shackle will not help.