There is no 6" suspension lift for the Xterra, they max out at about 3" for anything that retains much of the factory suspension components.

Fitting 36" tires is technically possible, there's one out there with them, but it requires very aggressive trimming of the sheet metal around the rear wheel openings and between the front tire and doors in addition to the suspension lift and body lift.

The only way to get 6" of suspension lift is to swap a solid axle in place of the independent front suspension to the tune of a couple of thousand dollars or more unless you can do the work yourself, plus a corresponding lift of the rear of some sort.
Another option that is of highly questionable benefit is the SLR "Stage 5" suspension lift, which I think I recall has actually gone into production recently:
http://www.xterraownersclub.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=73;t=000486
I'm not even sure if this kit provides 6" of lift or not, but it's the only IFS lift that could even approach it.

For my money, the solid axle swap is the only way to go. Keeping the IFS in any form for that kind of money is outright stupid for all but a rare few situations where it can be of benefit, and recreational 4-wheeling is definitely not one of them.

Brent