The shocks have absolutely no effect on the alignment in the Xterra, so get it done now and save your tires!
For that matter, the torsion bars don't effect it either if you got the ride height exactly the same as before. But, that's pretty hard to do and it won't hurt to have it checked out.
_________________________
ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy