IMHO whoops, yes, washboard, no, unless you're hitting your bumpstops.  That's when your axle sees the most force.  Coming down hard is the killer (and how many applications could you put that statement to?).  I made mention at the top, so I'll summarize here... Immagine yourself on a rollercoaster ride at your favorite theme park where you're not attached to the tracks.  Throw in an 8 mile section of 5 consecutive 30 foot floater jumps with 5 dudes in the rig at 80mph...  yup, that'd do it!!!!

Cougar, I too like desert racing, and rockcrawling. Somthing about pitching the back out around a big long curve in the middle of the desert just makes me smile...     I could see trussing it, cause bent tubes are definely no fun. In fact I was on the way up to my friends house friday, and as I was bouncing along his 9 mile long dirt drive way to the middle of narnia,  I started wondering how hard the potholes and massive washboard is at 50mph on my rear axle    His roads the closest thing i got to dirt racing unless i pack my ass up and head to Nevada, so I tend to have my fun, go fast, and get a lil' sideways... 
What kind of hard abuse did you put to the axle when you realized your tubes were bent?
Do you think some hard washboard could bend em?