[Gordo]

You need to determine which part is at fault, gears, cable or speedo head. Does the odometer still register and not the speedometer? If so than I would blame the head. If nothing registers, reach up behind dash and disconnect cable from head, pull it down where you can see it, go for a drive and take note if the cable insert is turning, if not you can remove the insert out through the cable (not always nesesary to replace intire cable/sheath) and inspect for damage. this insert can be replaced easily, (don't forget to lube it when you install it). If there is no damage to the cable, your problem may be at the trans drive or driven gears. Jack up and support the rear of truck (this minimizes fluid loss) removec cable, remove small bolt and clamp holding sleeve, then gently pull the sleeve out, this will give you access to the drivin gear, inspect for damage. Seldom does the drive gear get damage, but you can eye-ball it through the opening to see if it is damaged or not. To replace the drive gear would require removing the driveshaft and tail extension housing from the trans, not a big deal but probally not nesesary.