I've been curious what the spring rate is for the rear suspension on my stock '09 Access Cab. It has the original 3-leaf springs (nothing at all has been replaced or added in the suspension). I drove around with about 500 pounds of stuff in it over rough roads last December and never felt it bottom out, which seems to be completely opposite to what so many people here say.
I had 180 pounds of sand in the bed of the truck for the winter. I'm pulling it out for now...no snow forecast for at least a week. I took measurements from the ground to the wheel well lip at the vertical axle center line. The ground wasn't perfectly level, so this isn't a perfect test, but here's the measurements.
With 180 pounds of sand in the bed: 36 1/4"
With 120 pounds of sand in the bed: 36 7/16"
With 060 pounds of sand in the bed: 36 1/2"
With 000 pounds of sand in the bed: 36 9/16"
A linear fit to that data works out to a 600 pound per inch overall rear sag rate, or 300 pounds per inch on each side. This is within the range of what I think is reasonable for a normal vehicle. (Even just using the first and last data points it is 576 lb/in).
At that spring rate, loading the bed with the full rated capacity of my access cab (minus a 180 pound driver) would sag the rear suspension two inches. Nowhere near resting on the bump stops. What's our suspension travel in the back--five inches? Should take 3,000 pounds to push it down to full travel. I can sure see why it didn't squat or ride bad with a few appliances in the bed.
Someday I'll repeat the exercise on level ground with a wider range of weights and get it higher into the load range.