I ran into the exact same problem the OP has. Took me a few months to diagnose, due to it only repeating intermittently. First time it occured was when I performed an oil change. Completely stumped me. It wasn't until a few months later that I realized that my parking brake cables were seized in a position that once the brakes warmed up, they would drag. And I'm sure many of you know that when drum brakes drag, they don't continuously drag. They do a drag/slip motion over and over. Which causes that washboard effect the OP described. It was so bad that if I was at freeway speed(110km/h) I would have to slow down to below 40km/h to be able to limp it somewhere where I would be able to try and figure out what was happening. And of course everytime I'd stop, things would cool down, and it wouldn't repeat right away.

If he hasn't, I would change the parking brake cables. Cured my problem awfully quick.

 

If a brake is dragging, the wheel will get quite hot.