I had the same problem with my 06 off road when I had to adults in the back seat. From discussions at the time I got the impresssion that the springs were pretty soft that year. I, like many other owners added a leaf to the rear springs. It is best to add a long leaf as some have add issues with the leafs breaking if they added a short one. I found that my Xterra road better when towing after installing the extra leaf, and no more bottoming out over the smallest bumps.