I'm not saying you're wrong...but I replaced some 1" lift shackles from SLR with their H Braced 2" shackles, and I got a good amount of lift out of them...it actually caused the Xterra to sit a little ass high...and the difference between the old and new shackle was about an inch and a half as far as lift height.

But...at the same time..isn't it right that you don't want to go with too big of a shackle (i.e. a custom made one to give say 3" of lift) as that puts more stress on the leaves and can flatten them quicker?

SOA seems like the only viable solution in the end...if you SOA on a stock Xterra rear axle, you'll gain about 4-6" of lift after you're done...you have to modify the rear driveshaft (I think it needs to be lengthened by about 2" from the last I had heard to keep from popping out), and depending on how you want the shocks mounted, you may or may not have to modify or move those mounts...however, you don't have to get a Fronty rear axle to do so.