My wife's '09 4 banger AC, 2wd, 5spd is used primarily during the summer months for camping trips which involve towing a 2000lb pop camper with additional stuff in the pickup bed. This combination is no where near the maximum towing limit or payload limit for the truck but the vehicle sagged pretty bad. At the very least the sag wasn't equal to the amount weight on the vehicle, my 10 year old stock suspension Ford Ranger handled the static weight better. In an effort to find a solution WITHOUT lifting the vehicle I started looking for options.
The suspension TSB for 4x4's and Pre-Runners does not directly cover the base 2wd truck. However, the same ride conditions can be used with your local dealer/service manager to upgrade your suspension. I cited poor ride with weight in the bed, poor performance while towing, and the SAFETY issues involved during towing within the stated guidelines/specs.
Toyota has dumbed down the rear springs for the American market, 3 leafs, a very soft suspension. What Toyota won't tell you is they offer 2 other factory spring packs/rates, one for Canada and another for Mexico. The Canadian spring is 4 leafs with no noticable difference in ride height, i.e. a helper spring. The Mexican springs are significantly stiffer intended for daily hauling of materials/people while creating forward rake to the truck.
After discussing this with my service manager over a couple of months and my incessant visits I phoned in to Toyota and received a case number. My service manager was willing to go the extra mile, this upgrade must be approved by the regional service rep and my service manager had to make a handful of phone calls to set the ball rolling. In the end I had 3 or 4 case numbers all dealing with the same issue (DON'T LOSE THE NUMBERS). The Canadian springs aren't in American parts warehouses and have to be shipped from Canada.
Last week the Canadian springs were installed under warranty. Ride quality has actually improved slightly and hopefully when summer rolls around the truck won't sag quite as much. The goal was to have the truck tow confidently per the owner's manual listed weights, if I experiences any other difficulties I'll look to the aftermarket. I believe the trucks should come standard with the Canadian springs, we're not driving Avalons or Caddies, we're driving trucks.
For those 5 lug drivers who's vehicles are still under warranty and interested in the springs here are the part numbers:
48210-04560, Spring Assembly Rear and 48220-04200, Spring Assembly Rear -- Left
It goes without saying to treat the service manager with respect and don't expect this to happen overnight. Be persistent, without being a dick. Cite the problems and give the service manager the Canadian part numbers as a solution. My dad owns an '10 clone of my wife's truck, he has also had the work performed under warranty.