My speedometer drops about 20 mph whenever I hit a bump or joint in the road. For expample, when I hit a bump at 60 mph the speedometer will immediately drop to about 40 mph and then go back to 60. It seems that I have to be going faster than 30 mph for this to occur. Below that speed, the speedometer does not jump when hitting the same bumps. I have taken a quick look at what I think is the sensor on the rear of the differential and don't see any obvious loose or frayed wires. I have not looked behind the instrument cluster yet. The way it happens whenever I hit a bump would lead me to think that the problem is probably at the differential and not behind the dash. Any suggestions as to what I should look at next?