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Look for a commercial scale in your area, it's usually only $5-6 bucks (at least around here) to weigh in. Lower the jack and disconnect your truck so it's only trailer weight, no truck weight.
Load the trailer as you would for a race, so you get an accurate number.
But keep in mind I was running my Sporttrac at about 110% of Ford's rating for many miles. I even had the "evil exploding" Firestone tires on it for most of the time. If you don't think the truck is being overworked, and you only pulling 70-80% of Ford's rating I wouldn't lose any sleep over it. Change the fluid annually if you want to be sure, then you can find problems before they get serious.
I have an 06 FX4 and i tow alot often with my truck... Have 35 inch tires change the oil every 3k had the trans done 2 times now and i have 75k on the truck... I just added F250 rear Leaf springs to it today. I had helpers on there from JC Whitney and they were junk. So i'm hopping the springs will help me there. I tow a 18 ft car trailer and a 30 ft toyhauler with 2 big lifted quads in it... And no problems or worries here. 8K is fine for your truck.
Two suggestions, get the 17 inch tires, they will make a big difference (from 20 to 17 would be almost like going to the next bigger size gears in the rears!)
Invest in a Anti-Sway, leveling hitch! It will make your ride much safer and smoother, your truck wont squat much at all then!
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.