roff

My buddy and I were out in his '08 Cherokee, 15 miles from pavement, on a dirt road that is pretty good if it weren't for the never-ending washboards. We had been driving on this surface for about 25 miles (we were on our way back home). We were going maybe 30MPH when we suddenly heard, and felt in the floorboard behind the front seat, this loud, rapid thumping. The rate varied with vehicle speed, going away when we slowed below about 5 MPH.

He stopped the vehicle, and I crawled partway underneath, looking for something wrapped around the drive shaft. There was nothing visible. He started the car again, and we accelerated slowly. The thumping resumed. We were starting down a hill, so I suggested shifting into neutral. the car continued to accelerate, but the thumping stopped. We stopped again, left the tranny in neutral, and revved the engine. No thumping. He put the tranny back in drive, we drove off, and covered the remaining 15 miles of washboards with no more problems.

What the heck happened? Have any of you ever experienced something similar?
I am convinced that the thumping had something to do with driving on washboards for maybe 25 miles, but what caused the thumping, and why did shifting into neutral and back into drive cure it?