My speedometer needle pegs at the high end of the scale (ignition on, engine not started)
or
it swings from stop to stop across the entire scale (any speed)
or
it vibrates over a 10-20 mph range (not related to actual speed)

I know this has been discussed before, but my problem may be different because it's happened before and it always went away before I got a chance to diagnose it. (hard to find a problem when it's not there)
I'd rather fix it than wait to see if it fixes itself. Where should I start looking?

 

Time to start checking the grounds......

 

Hey Duk, can't speculate on the engine off part, but when my speed needle waved around like that it was on the mechanical side, driven gear depth on the drive worm on the inside (tail shaft in my case). By rotating the driven shaft journal clockwise or CCW about ten degrees it'll get worse or better so proceed accordingly. I adjusted the journal as fol: loosen the 1/2" cap screw that holds the retaining yoke, pull the yoke out of any grooves, and rotate the whole assembly using the ratio drive and VSS as a handle.

 

Give a look over on your wires, sounds electrical to me, with the motor off.

 

Def. electrical if you have problems with the motor off. I would look for a shredded wire, or a bad ground. Mine was screwy for a while and it ended up being the pigtail. As said I would check all grounds, and then follow the wiring harness back up towards the front of the truck and look for any bare wires that could be grounding out.