The units that operated as convoy escort were Transportation Companies and only authorized the trucks. The M113s were scavenged from ones that had been damaged, as were most of the weapons. The truck cabs were armored with armor plate on the doors and around the windshield area.
Here are some actual ones.
The versions with M113s on the back were the last iteration of them. They started out with simple boxes made out of armor plate on the back and a couple gun stations.
More info on them can be found here: