Later model (read: metric) automatic transmissions from the era of our cars are typically not all that strong. They were good enough for the relatively low-powered engines of the day, but over time even those motors can give the transmissions a real workout. So when your ride suddenly starts shifting strangely—missing a shift, delayed shifting, the inability to reach overdrive gear, etc.—it's natural to assume that the transmission is shot. But if you're lucky it could just be a US $2 part.
The TV (throttle valve), a.k.a. kickdown, cable attaches to your throttle body. (Some cars have rod linkage instead of a cable; this still applies to you). Its purpose is to downshift the transmission during deceleration, and control upshifts on acceleration. There is a plastic bushing that mounts the cable to the throttle body plate. When it breaks, your cable (or linkage) becomes detached, and you get funky shifting. Now this is not a good situation for you, because riding around without a kickdown cable can cause immediate internal damage to the transmission. The new bushing kit is roughly US $2 at nearly any good parts store and should be in stock since this is a common occurrence. Or you can order online at places like LMR (Ford dealers have it under p/n F3SZ-7H303-B). You may need to drill out the hole in your throttle body lever to ~3/8" for the new cable to work, depending on your model year. In a pinch, you can tie up the cable with a plastic Zip-tie or wire with satisfactory results. Keep in mind that this is a potentially serious problem and should be taken care of ASAP. This problem is most associated with the AOD transmission.
So what if it's not this little part? Well, if you have a C5 (or transplanted C4) there is a modulator valve on the transmission casing that can go bad. Also, there will be a vacuum line attached to it, as this controls the line pressure. Changing the modulator usually helps, although you cannot discount a cracked vacuum line. In any case, that fix would be rather inexpensive as well.
If you have the AOD, however, things don't look too good for you. In general, Ford transmissions simply go at their own discretion. Doesn't matter that the day before it shifted perfectly; it can simply go out on you at any given moment. This can happen anytime but most prominently after 100,000 miles. So should one live in fear of the transmission going? Well, if the car's shifting fine and you actually remember to change the transmission filter and fluid on a regular basis, then you'll be okay. But just try to keep that in the back of your mind...and in your bank account. As always, it's recommended to see a professional and get their opinion too.