Remember that not all replacement engines or transmissions are the same. There is a big difference, both from a price standpoint, and from a reliability standpoint of going with a quality remanufactured engine or transmission, versus a used pull from a junkyard.
If the Ford dealer is offering to install a genuine Ford remanufactured transmission with a Ford-backed warranty for $6600, I’d strongly consider that if the rest of this truck is in good condition. At today’s prices, you can’t buy another used truck with 125,000 miles for that amount of money, and $6600 is less than a year of typical loan or lease payments.
Otherwise, I’d have the truck towed home to my trusted mechanic, and have him repair or replace the transmission.
I would not use a “random tranny shop” that is 400 miles away from home, unless they are proposing to install a brand-name remanufactured transmission with a decent warranty, such as Jasper. A used transmission from a junkyard? Forget about it.
And once you fix this truck, I’d keep it for as many years as possible. An investment of $6600 might seem like a lot of money, but at today’s prices, as long as the truck lasts another 2 years, you’ve covered your costs, and every month after that, if you keep it, it will put money back into your pocket.