I like the way Yardsale thinks. I went through a similar thought process when I bought mine last winter. The choices were $8000 for any one of several Sport Tracs around here with mileage from 100,000 to 133,000, or $11,000 for the one I bought with 63,500 miles. Both amounts were more than I intended to spend when I started my search, but I figured I could spend more now and drive it for 7 or 8 years before it got as many miles on it as the others have now. I kind of amortized it out, and came to the desicion overall the lower mileage truck would be cheaper than the others if I had to find a replacement in a few years. I tend to wring all the miles I can out of my vehicles. As I have learned from the forums, the Tracs I passed on are about due for some of the common problems to start cropping up, so in the long run I'll probably save a ton of $$.
In your case, Marsha, your truck is paid for and you like it, so doing the repairs will be cheaper in the long run as opposed to buying a different vehicle. You didn't really think a bunch of Sport Trac owners would tell you to get rid of it, did you?
Years ago I was trying to decide if I should fix the rust and repaint my '79 Ranchero, or sell it and get something else. A friend I called for advice said "You'll know if you made the right desicion in about a year and a half." I sold it. Saw it a couple of years later and knew I'd made the right choice. By the way, this friend is an outstanding Ford mechanic and I used to drive 45 miles for him to work on my cars. We're 90 miles apart now, so I just call him when I need help and he walks me through it.