I find higher speeds work for a short distance, but when the washboard frequency/amplitude changes, then things get out of control and easy to go chattering off into the bushes.
Like Blmpkn stated above, the front weight bias responds differently than the unloaded rear end.
+1 air down
My Toyo OC A/T III LT tires are normally at 37PSI which is bone jarring on washboard at any speed. Down to 25PSI makes a big difference. Toss in rocks and longer drives, then I run 18-20.