I read on another forum that the last generation guntrucks in Vietnam were based on M35A2 trucks (if the weren't based on the larger trucks), and they were painted black instead of olive drab. Can anyone tell me more?

I don't know about the "last" ones being based on M35A2's, but some of the later ones were. It is correct that they were painted flat black as well. Most of the grunt-built guntrucks were repainted flat black, and not green.

The last Vietnam era gun trucks were built on the M 54 five ton chassis and not the M 35 A2. There were M 35 A2 gun trucks out there (Maverick was one). All the trucks of the 8th Trans Group were painted black with the yellow nose paint. At the very end all graphics were ordered removed and were all transfered to one consolidated unit (late 72 early 73). The main reason that most of the 2 1/2 tons were phased out is the weight factor. They could not withstand the additional weight of the armor plate. Other units had gun trucks, or security trucks in their units. These were usually paint regulation colors and not black. The black trucks were built primarily for convoy security duty while others were designed and built for bridge, perimeter, and fire base security duties. There were a lot of 2 1/2 tons used for this kind of duty. They had a lot of these equipped with the quad 50 set up like the model Nancy by AFV Club. The Air Force had gun trucks for base perimeter security duty as well. They were armed with M 60's and or M 2 50 cals. the Engineers units even had some of their five ton dump trucks converted to gun trucks for road security to protect the crews on mine clearing duty.