I've noticed since I joined the forum that this topic seems to come up pretty regularly, not surprising giving the age and mileage of our trucks. Cables and gears are always worth a look when the speedo goes crazy, but often it is the instrument itself that is causing the issue. I'd like to pass along a procedure I have used to rehabilitate noisy and jumping speedometers. Basically, the speedometer works by the cable from the tranny spinning 2 magnets around a drum attached to the needle. The faster the magnets spin, the more force is applied to the spring loaded needle(spring loaded to return to "0"), and that force working against the spring stabilizes the needle at whatever speed you are traveling. On the back of the speedo where the cable connects, is a gearbox. That gearbox works the mechanism that rolls the odometer. There is a bushing in there rather than a bearing for the cable, and over time the lube dries out, causing the needle to bind, jump, and make ugly noises. This might appear more often when it's cold and the lube is thick. If you remove the instrument cluster, you can attach a small rubber tube to the speedo drive, connect the tube to a drill to allow you to spin the mechanism, and while you spin the speedo you can apply a light machine oil to the bushing. This should loosen everything up and get the speedo happy again. If you are so inclined, you can run the drill in reverse and roll back the indicated mileage(of course, this is only to remove any mileage racked up during the drill spinning process). Pics below are of the process as performed on a '73 Dodge van speedo, I have also done it to the speedo in my '70 AMX.
Here is where you want to attach the rubber tube and apply the oil ...


Here is the hose attached to the drill and the speedo.


That's it, hope this helps some folks save a little cash and a little labor chasing a jumpy speedo.
Pat D.