I expect Desertman's correct. Same thing happens in snow. The give-away is that your tires were 'shaking.' Anything else (e.g. Engine knock) and you wouldn't have had the tires bouncing around.
At 18 psi, you've got wayyyyy too much air in your tires. Drop down to 12'ish and you'll get a ton more grip. Need to be mindful of turning too hard, but driving with care it shouldn't be a particular concern.
Drop to 12psi, put your lockers on front/rear, in 4Lo, and you can drive around dunes all day long w/o issue if you're someplace like Michigan's Silver Lake Dunes.
The thing with dunes, is it may 'look' smooth but they aren't so smooth you want a ton of speed. Yes, need momentum climbing a 45 deg dune, but too fast and you bottom hard on the imperfections. Also, go too fast, a lot of dunes drop off abruptly (as in razor's edge) at their crest. You need control as you crest the dune. Plus, most folks don't have suspensions geared to support flying through air<g>.
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