Crazy vibration on the highway..

Hey everyone. I was heading down the highway yesterday and the truck was vibrating really badly, to the point where the steering wheel was shaking all over the place. I pulled over to see if I had a flat or if a lug was loose or something. Everything seemed ok. I know normally this means a balancing/alignment issue, but I had it done 2 months ago.
The strangest thing is, after pulling over and getting back up to speed on the highway, the vibration stopped completely. It went back to normal.
What the heck could have caused this temporary violent shaking? I'm concerned it could have been something with the engine or drivetrain, because if it were a tire issue, it wouldn't have went away on it's own. Any insight is appreciated, thanks.

 

Been driving in mud recently? Have any packed in your wheels?

 

No mud or ice. At most there could have been stones picked up from an unswept parking lot I was in; but I doubt a few big rocks would have made that much vibration.