When did the term "technical" come to denote a converted pickup truck with an added weapon mount in military jargon?
According to counterinsurgency expert David Kilcullen, as quoted in an article in Newsweek, the term originated in the 1990's in Somalia.
Indeed, Africa, says Kilcullen, is where the truck got its nickname as a fighting vehicle, “the technical.” “When [nongovernmental organizations] and the U.N. first went into Somalia,” he says, referring to a period in the 1990s, “they were not able to bring their own guards. So they got so-called ‘technical assistance grants’ to hire guards and drivers on the ground. Over time, a ‘technical’ came to mean a vehicle owned by a guard company, and then eventually to mean a Hilux with a heavy weapon mounted on the back.”
The term's been around since the Somali civil war during the 90s. Wikipedia bears this out:
The term technical describing such a vehicle originated in Somalia in the early 1990s. Barred from bringing in private security, non-governmental organizations hired local gunmen to protect their personnel, using money defined as "technical assistance grants". Eventually the term broadened to include any vehicle carrying armed men. Technicals have also been referred to as battlewagons, gunwagons, or gunships.