I had the same issue, especially when towing stuff. Turns out the stock springs on our early-gen R50s are just too weak. I installed a kit of air helper springs in the rear coils for about $80. Here's XPLORX4's installation instructions: http://xplorx4.com/pathfinder/upgrades/airlift/I've also heard of installing heavy duty coils, but I can't find a link to them at the moment.