Oh, wow. I went back and watched Jimmie's onboard. It was, in fact, a rear gear failure. About a lap before it failed, there was a lot of fluid on the rear bumper camera, and you could see a bit of smoke trailing behind. Then he reported the smoke in the cockpit, and half a lap later, it made this godawful shrieking/buzzing noise and he said "it's laying down... it just quit." He sounded kind of deflated, but Chad was very matter-of-fact about it, and certainly neither sounded frustrated.
With the penalty, interestingly, Johnson was completely silent on the radio. I would have expected one of his trademark "ARE YOU :censored:ING KIDDING ME?" radio transmissions.