How much weight will be in your Xterra? I tow a travel trailer that is 3500 lb empty. Loaded up with gear and water I figure I'm over 4000, probably closer to 4200 or so. I then add about 350 lbs in the Xterra with passengers and cargo and I've got about 500 lbs of Shrockworks and Gobi gear and heavier tires on the truck. The max GVWR is 9658 lb and I figure I run pretty close to that. I've towed over Vail Pass and through the tunnel. Last time through was west to east (which is tougher than east to west) and had no trouble maintaining 45+ mph. I've also towed over Monarch Pass which really taxed the rig, but we made it. Just realize that you won't be able to do the speed limit uphill or on level ground and you should be OK.
The rule of thumb is that you lose power at 3% per thousand feet of elevation gain. Eisenhower tunnel is at roughly 11,000 feet so you have about 2/3 the horsepower that you'd have at sea level. My daily drive is at 6000 ft so I live with only 80% of the power of you guys at sea level.
Well aside what has been said, Hope ya have a equalization hitch that will help a LOT. Use higher Octane fuel as it will help on the hills. Be tuned in to what is going on with the brakes on a down hill, may want to gear down and stop and let them cool if you see any puffs of smoke. If you go the same speed as sane truck drivers do, you'll be ok. There should be signs as to the limit for trucks down hill, follow that. I'd do this in a heart beat and not worry about it, I have towed like 3K lbs above what we can tow, but then I know the what's and why's. So that's to say it can be done although if you biff the factory is out of the picture as far as liability because they "Told ya so" Just don't get into a hurry. Use the gears on the tranny, don't lug it. MC
my wife and i just towed an extrenmely overloaded 12 foot uhaul from cincinnati to seattle by way of the colorado rockies, through jackson hole, and over the teton pass. we made it with no injuries...except when we forgot to block the tires when we unhooked it one time and it went soaring through out campsite!
Should be fine, I towed basically the same thing as you (U-Haul tandem axle trailer and a '89 300ZX) the total weight came in around 5600lbs plus three people in the car. The problems I had were the position of the car and the rear springs on the X. The design of the trailer position the car way to far forward and placed most of the weight in front of the front wheel of the trailer. In retrospect if we had backed the car onto the trailer it might have balanced it a little better. The rear spring on the X are just too week by themselves and rode on the bump stops for the trip. It was only around town, but if I would have to do the tip you're doing I would do something with the rear springs. Other than being on the bump stops the X really didn't care about all the extra weight. Then again this is all at sea level. Acceleration and breaking were not nearly as effected as I was expecting. With stiffer rear springs I would feel fine about towing that weight for long distances.