Last week I filled up the back of my FX4 with firewood - I think about 1/3 cord's worth. I don't know how heavy the load was but the firewood was wet and heavy. It was enough to make the back sag a little. Actually it looked leveled out (as opposed to leveling the front end).
It was my first heavy load so I wasn't sure what to expect. I was surprised that I didn't notice *any* power changes pulling out of the muddy lot. Like nothing was in the back. The only changes I noticed was accelerating at 50+ and I could tell the brakes were working harder. Very impressed. I love these trucks!

 

I feel the same way. I have a 4.6L and just recently I helped my sister with her bedroom set. The total weight was around 800lbs and it seemed as if it the truck was empty. Gotta love it.

 

I hauled a bunch 2X4's and about 2400lbs in cement when we were building my buddies fence. Not sure how much the wood weighed in at but it was completely filled to the top of the bed and then added about 30 bags (80lbs a bag) on top of that.Had to be at least 5000-6000 lbs.
Rear end sagged like hell.

 

Maximum payload is from 1340 lbs upto 3000 lbs, depending on your options, thats why it sagged. I see trucks all the time with a lot more weight that this and the rear end sags really bad and probably dangerous. Towing\Hauling is different, maximum is from 6,650 lbs to i think 9,000 lbs, depending on options.