MountainBiker, I have said this before that the Mexican gangs know how to get around the rural areas and in there countries the Central American gangs lived in rural areas. I read in a news article about a year ago that these gangs MS 13 and the Latin Kings have more guns and rifles weapons then the military.
I do think that in a SHTF the rural areas will have gangs going into them and will they also go into the jails to get there members out? I think so. You are right that the guards will leave in a SHTF time.
The cities will also have a war with these gangs but they will make it a point to go to these jails to get there members out.