Hey dude,
I dune my 4runner a lot without the top. I end up with dust EVERYWHERE, but not that much sand in the back. Most of the solid sand comes in on the bottom of your shoes or from whatever else you throw into your truck. Lots of dust inside is unavoidable, unless you keep the roof on and the windows and vents closed.
Have fun! B.

... something to get you in the mood...
http://www.pbase.com/brianmoore/image/26234916