Steves response made complete sense. I fear that you are experiencing a short respite from the problem. A speedometer not working and then jumping up to 20 is not normal. When you add that it also hadn't shifted then you can see how that nut must be loose. Four bolts to remove the driveshaft from the flange(You may have to remove from the diff too, but Steve didn't say so) then stick asocket on an impact wrench and tighten things up. Good insurance and 1/2 hour tops invested.