The popular mod is installing Airlift 1000 air bags in the rear. They run about $75/set on Amazon.com. There's a thread in the 2005 Pathfinder section with lots of comments from people who use them. Most seem to install individual lines to the bags as opposed to a shared line with a "T" fitting and they usually run them at 10 psi under normal driving conditions, but you can inflate them more, of course, when towing. R51's have issues with bottoming out and sagging when towing; these will fix both of those concerns.
Here's the thread:
http://thenissanpath.com/viewtopic.php?t=3219
Another thing that helps is upgrading the rear shocks absorbers. A set of Bilstein HD's or a monotube shock like Monroe Reflex or KYB Gas-adjust make a considerable difference in the ride quality.