It sounds like the previous owner may have beat hell out of the truck and then traded it. Does it have signs that a tune was installed? A bad tune can roast a transmission in short order, especially combined with oversized tires.
Did the dealer provide a quote to repair the transmission? These units are fully serviceable; any experienced Ford transmission tech can disassemble and reassemble the unit in less than one day. However, if there are metal shavings present in the fluid or internal components have been sitting around unprotected and exposed to contamination, I would replace the unit instead of risking future issues. The Ford/Motorcraft remanufactured unit is warrantied for 3 years/unlimited miles, but the truck will need to be running the factory calibration and factory size tires.