IIRC, isn't the GCWR on a Max Tow F150 something like 17,000lbs?
At 21,480 lbs you were almost 4,500lbs overweight, right?
That's crazy, no wonder the truck wanted to put you in a ditch.....it was probably mad at ya.
That GCWR is actually right around what I am with my F350 dumptruck. I pull a drop deck trailer with a Gehl skid steer.
Trailer weight is ~3,300lbs
Skid steer weight with bucket, forks, and a full tank is ~9,000lbs
Truck weighs ~8,500lbs.
Total GCWR is just over 21,000lbs with me in it and nothing in the dump bed, and I'm just under my max. I couldn't imagine exceeding it by that much, never mind doing a hill climb with it.
(Then again, my dump is a '94 that had all of ~215hp/~425tq when new.