The check engine light on and the transmission shifts hard?

Last year, my husband took my SUV on a hunting trip. He drove four hours (all highway miles). He exited the highway (and came to a complete stop). He pressed the accelerator to go and when it shifted from first to second gear there was this real hard shift (Like there was some sort of transmission problem). After the truck sat for a few days, it drove like nothing was wrong with the transmission. When he exited the highway to our home from the four hour trip the truck shifted very hard again from first gear to second gear. Only, this time, the "check engine" light came on. Now, if I drive half an hour (on the highway) this hard shifting from first to second gear sometimes happens and sometimes doesn't happen. The "check engine" light hasn’t come on in a while. If I shut the car off for a few minutes, then restart the truck, it's usually okay. Is it a sensor or what?