The speedometer is stuck at 120 and the radio.

You blew a fuse for the radio. That's common from the surge of connecting jumper cables or battery chargers.

The speedometer jumped at the same time. They are not spring-loaded like the extremely reliable speedometers of years ago. The pointer is looking for the shortest way back to "0" and right now that is clockwise. Drive fast enough for the shortest way back to the actual speed to be counter-clockwise and it will go there, then follow the truck speed back down like normal.