[SCOTI]

My observation has always been 'low-back' buckets were in earlier year models & then 'high-back' buckets in later years. Kind of the whole progression as far as the safety aspect (lap-belts to shoulder-belts as well). High back buckets were probably safer as far as neck injury in rear end collisions. They were optional the whole time until the later end of their production run when they were mostly used in Blazers/Burbs. Them not being as common in trucks for the final years (81-87) could be because of corporate mandates (build option 'group' packages) becoming increasingly restrictive vs. earlier years when you could check off select options & wait for exactly what you wanted.