My truck is a 92 with speed sensor, no physical speedo cable. When the turn signal is on, the speedo bounces continually in about a 20mph increment, keeping time with the clicking of the signal. The truck is completely rust- and corrosion-free and I can't find any evidence of poor grounds. From what I've read so far, the most likely culprit is the flasher relay, part# 81980-12070. Is that the general consensus? Thanks for the input!

 

The flasher is not an expensive part and is pretty easy to replace. So there's not much reason NOT to replace it.

But my money is on the speed sensor. More commonly, you'll see the speed indication drop to zero, then after few seconds come back. RockAuto has after-market parts for not very much money. Personally, I'd pull the old one first so that I could match the photos, but RockAuto seems to suggest it's just manual v. automatic.

Good luck!