I've seen jeeps going 4mph over rocks blow u-joints and send shrapnel at people standing around...I have no doubt in my mind that a stock diesel truck makes enough power to blow a u-joint when pulling a sled...and when a little kid standing by takes metal to the head and dies the rules will really get strict.
I used to ***** about the rules all the time when I didn't want to build or buy the safety stuff. I now drive my truck on the street everyday with full U-joint shields and loops, I even had to take the bottom of one of my loops off last week to haul 3200lbs of concrete because the DS was rubbing the shield.
I pulled in a street legal class friday night where helmets weren't required, yet I still had mine on, only person in the class who wore one, but if something catastrophic decides to break, I wanna be able to walk away without a piece of metal in my head. Thats just me tho.