Clonking noise from underneath on very bumpy ground.

Jeez I was trying to make that title sound a bit more technical and precise but that's what came out and it's at least accurate.

I changed the oil (05 plate 85K miles) and it was clean as a whistle, very surprised to see no crap on the drain bolt and to celebrate I went for a spin off road, did a few miles up over the hills to check out a loch I've been meaning to visit and no probs at all. Then on the way home I took a very rough track shortcut which caused quite a bit of body roll. On the bits that caused the most sway I was getting a pronounced clonk which I think corresponded with the sway of the cab and only on the very worst bits.

My first throught is a missing bump stop but can anyone think of anything else to look for while I'm under there?

PS: Forgot to take a photo, must do that next time.

You might want to check your anti roll bar bushes and links. If you're really unlucky you could have a rotted out body mount like I have.

Have you got a tow bar fitted? Some people have reported the exhaust banging on it.

The clonking I've had and the causes, drove me mad finding them:

Rear exhaust silencer hitting tow bar (if fitted) as Soeley said.
Rear spring lower location cone, I suspect rust was stopping the spring sliding over this producing a knock with a bit of bong thrown in. Lifted car to unload spring, wiggled the dirt free and applied spray grease - problem gone.
Worth a check.

Cheers guys, no towbar but I'll have a good look at all those other things.