Anecdotal info
An old friend of my Dad's saw my Bronco in the garage one night and came over to talk about it. He apparently drove one in Vietnam, and he hated it: he said that the damn things had the hardtops and no ventilation, and were hotter than hell. He remembers they rusted quick in the jungle (!) and that every PFC would beat the hell out of them. Still, I had no idea prior to that they were used by the military, mostly MPs in his experience.
Also - an early post noticed that it seems broncos weren't made for a while in early 1968. As a Mustang nut too, I know that Mustang production was halted in early '68 due to a union strike. I bet Broncos were affected by the work stoppage, too.
Just my .02