In 04 I bought a 36' MH and towed a Jeep. My 00 Nissan P/Up I had at the time wasn't towable. Even Remco didn't make a disconnectable driveshaft for it.
IIRC, I don't think the Xterra is listed as towable either. Check on the Remco website to see if they either sell a driveshaft disconnect kit, or an auxillary transmission oil pump.
All else fails, you can always use a drive on trailer. The upside of this is you'll have the ability to back up, something not possible when towing 4 down.
Was camping the weekend before last, saw a big Monaco MH towing an H2 (HEAVY).