ABS, Brake and traction control alerts.

For several months I have been getting C1249, C1380 and C1391 and my ABS, Traction control and brake light come on. They will go back off for a while (maybe a few minutes, maybe a day) and then come back on.
When this started I was driving on the highway and heard a sharp clunk under the car. I assumed I had hit something but I did not see anything in the rearview. Then I noticed all these lights on. I stopped and shut the car off and restarted, but they remained. As I drove they went off.
When the lights are active the brakes do behave more sluggish, but hardly noticeable.
I am thinking this is a damaged speed sensor, but I don't know how to test them and I don't have a good way to get under the car.
Any suggestions would be appreciated.