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.