I am in the keep it fix camp.
I have a 1997 f 150 with 200k on it and the transmission went south. It was $2000 for a rebuilt installed, I just couldn't come to grips with spending $2000 on a truck that was probably worth less than $3500. After looking for another used truck, that were more roughed up than mine, and the price of a new truck, just couldn't see the sense in that either. I went ahead and put the new transmission in and 20k miles later its running great, and I am really glad with the decision to keep the old truck.
If you do decide to buy a different truck, I got a 1999 F 250 7.3 with 148K for sale.