Every time I hit a good bump in the road the speedo quits working for a second or two and then goes back to working.

Any ideas of where to start looking for the problem would be appreciated.

Thanks

 

The speedo on that truck gets its signal from a sensor on top of the rear differential. Firstly, have a look at that sensor for visible damage. Unplug it and check for corrosion in the terminals, then put it back together with a bit of dielectric grease. Follow the wiring from the connector to where it jumps over to the frame and heads forward. I've seen many times where that wiring has become damaged by years of gravel spray. Those are the biggest problem areas, hopefully you find something there.