Chuck, if you have the speedometer removed from the truck it is not difficult to remove the speedometer mechanism and clean and re-lube it. A few years ago I posted instructions on how to do this. The main and most important thing is to be really careful removing the needle. The needle may not want to come off and you may break the shaft removing it. You can isolate the needle with very thin pliers or a hemostat, but even doing that won't guarantee you can avoid problems.
But then I came back and explained with images how you can repair/restore that broken shaft using easy to find brass tubing from K&S Co. Sadly we're not adding to Tech Tips these days, but maybe you can find my post on repairing the shaft and oiling the speedometer. There is also a post I made on renovating the odometer wheels (along with free printable images you can use with waterslide decals or double-sided adhesive) and instructions on how I do that.
Most of these speedometers don't seem to be worn out nearly as much as dry and lacking lubrication. The ones I've seen simply need to be revived a bit. Sorry, but I can't recall offhand the size of the thumb nut. Offhand I think it is 5/8"/18 thread at the speedo head but you should check me on that.
