All you have to do dude is go get a Magnaflow or Flowmaster exhaust, take it to a custom exhaust shop in town, and then have them put on duals.

Now, if you want it to come straight out on the back, I don't know about 05' XTerras, but you would need to remove the spare on anything lower. The other option is to have the exhaust tips come out right behind the rear tires, but the spacing is tight. I have seen ONE XTerra where it had duals and the pipes came out right BEFORE the rear tires. Really up to you man.

I have a Flowmaster 40, 3 inch exhaust tip, 2.5" exhaust pipe, and it sounds loud and great.

-Seph