Ok, this was drive error. I tried to blame it on the tires but it was just a silly mistake. It was at the end of the road and I saw Rocky stopping so I slowed down. That meant I didn't have enough momentum to get up over the little rise on the right side so I gave it a little gas. All four tires responded to gravity and I slid unrecoverably to the left winding up next to a small creek. Front and back downhill tires had no traction at all. A slight tug from Rocky and we were on our way again.