so, just made a drive from houston to austin, and halfway there,
the speeedometer needle starts bouncing all over the place.
once i hit about 70, it was fine, slow down and it bounces all over then sets itself. should i be worried? should i look at getting fixed?

 

What is the year, make and model of your vehicle? Is the speedometer electric? Or is it mechanical?

 

1996 b2500 cargo van
has to be mechanical

 

The service manual for my '94 B2500 shows that it is electrical. There is no speedometer cable attached to the unit, only electrical wires. I suggest that you get yourself a factory service manual.

 

There are some threads for similar issue as yours can be found and read a bit using the search bar. Seems the likely culprit could be the connection to the output speed sensor or the sensor itself.
It's located on the tail of the transmission with wiring to it, easy to replace, and think it's under $20.
Could just try inspecting and cleaning that connector first