A golf cart with an expended LAW tube, welded to the back, and used to launch as many motars as it can before one blows inside and kills it's operator, is a technical.
Any motorized civilian vehicle augmented to kill shit in war is a technical. Except maybe VBIED's, that I'm not sure about.
Which means that's the question you should be asking: was a technical used in Oklahoma City bombing?
The only place the OK City bombing was considered a war was inside Timothy McVeigh's diseased mind, so I would contest that his rental van was not a technical, but mostly on a technicality.
Since when do both parties have to agree there at war, for it to be a war?
McVeigh and his fellow militiamen, or conspirators, felt they were at war with US government.
I'm sure there's countless people living throughout the Horn of Africa, who feel they are at war with the US government, while the US government would deny that wholeheartedly.
Does that mean any of their pickup trucks with M2 Browning's mounted in the bed, are not actually technicals?