True about the tires.
Theoretically speaking, I think when I was rocking that what was happening is the ears on the frame were bouncing on and off of the hard rubber stops which would quickly transfer the load to the other side. Then this kept happening until I let off the throttle and it would settle down and straighten up immediately. Again, theoretically speaking, if I get the truck leveled up and off of the bump stops, this wouldn't happen.
Where this theory can go south is in the case where the bump stops may have been giving me some sort of simulated overload stability. Food for thought.