Not an in depth guide to where they are but sprinkled all over the Sinclair Parish are flat bed style trucks pulled to the side of the road with either logs or barrels on the back. Shoot them when you are close enough after the triangular crosshair prompt appears by clicking
to release the "trap". Just wanted to clarify the im 99% sure you must actually obtain a kill off triggering the trap... I must have hit 20 of them now and no cheevo.
EDIT: This may also fall under the category of cumulative achievements for this game that are bugged, which would be REALLY stupid seeing as this DLC came out not even 24 hours ago and myself and many other TA faithfuls have been flooding 2k with support requests for bugged achievements that were in the original 1000GS.
EDIT: Multiple other players reporting this achievement as gitched, seems hit or miss.... thanks again 2k