Try turning all the way to one side, then go over a small bump. If it is the same noise, it is probably the steering stop. But if it happens on on ramps, I dont think you will be turning full lock going that fast. Check your exhaust to see if it is hitting something. Some have had problems with the pipe hitting the torsion bar bolt. Check your transmission/Tcase mount to see if it is moving around a little. There is a metal "loop" hold down thing for when you dont change out the mount for 15years it will still stay in place when the rubber is no longer attached. Or T-case roll stoper bushing and mount. Or just get under there and start shaking things with your hands and see if you can see anything moving around. \

Whatever you find, let us know.