kind of strange you made the post on Jan 31, 2013 since I rode through Guerro Negro that very day and got gas at the PEMEX north of town.
Today on my way to La Paz I was smoking along that straight stretch south of Ciudad Constitucion , came over a slight rise before a bridge, 4 cows 2 on the bridge and 2 just before. Lots of heavy braking and passed them at 25mph.
I remember that poor guy Clayton that hit the donkey and his sad story. Glad to see you survived
Ride till you can't - Marty Hill