Believe it or not, mine towed great. I needed the 4.10 gears and the 6 speed, and it was pretty hard on it, but it was remarkably stable. A 3.6l with decent gearing would do fine.
You are right in theory that leafs are better than coils. However, a properly set up 5 link, like in the JK and the Ram 1500s does fine. My WJ, on the other hand, had a weird modified 3 link set up and it SUCKED for towing till I put an Addco rear sway on it. That helped dramatically with towing and handling, but it made it ride stiffer. Surprisingly, it didn't make a noticeable difference in rear flex.
I've wondered before if you put a performance rear sway bar on the Xterra if if would cut down on the rear suspension bottoming out. I'm not an engineer, but with how much it stiffened up the rear suspension on my WJ you'd think it would help and definitely make it handle better.