I think a good model to look at for real world capabilities would be the "up-armoring kits" shipped to Iraq and Afghanistan once IEDs were recognized as THE threat.
Units in theater did a lot with existing materiel. At least the Marine Corps responded pretty quickly in developing and shipping armoring kits (I'm not saying the Army didn't, but I know the USMC did).
http://www.globalsecurity.org/milita...ground/mak.htm
"Strapping" on extra armor to an already existing fleet of HMMWV or other vehicles is 'easier' then producing a whole new fleet. I think a lot of hillbilly armor would be used by stateside units. Some of that hillbilly armor might be applied to 'standard' commercial heavy duty trucks (like FORD F250 or larger).
Look at what the Mexican Narco Cartels have been able to produce in underground facilities. I'm certain that many US cities/or states could produce something at least as good. The USMC depots at Albany, Ga and Barstow, CA would be able to produce some interesting vehicles since they store a LOT of semi obsolete kit and have a very good supply of machine tools, skilled machinists and 'stock materiel.'