That's not entirely true.
Before any of these vehicles came along, General Motors - Chevrolet Division made the Panel Truck aka Sedan Delivery. I know, I still have a 1967 in the field behind my mom's house. It was my first vehicle and served me well for many years.
My guess is that the sedan delivery truck came out after WW II - 1946 and that they came in many different styles. The style with the windows and the 4 doors was called Suburban and the ones that did not were called Panel Trucks.
The 1949 Panel truck was the basis for the Monster Truck - Grave Digger.
My truck is a 1967 and has no side marker lights, seat belts, or parking turn signals in the grill that burns when the headlights are on. The door frames at the top of the windows are square and not round like the pick up trucks.
The back doors, which were nicknamed "barn doors" were also square.
The second most famous truck was a 1970 Ambulance used in the hit tv show Emergency. That truck was followed by a 1971 or '72 with the plastic egg crate grill and was also a yellow ochre and white lacquer paint job.
Here is a picture of the 1969 Chevy that they used on the origional episodes -
http://imcdb.org/vehicle_132407-Chevrolet-Suburban-1969.html
If it has 8 bolt hubs, it is a 3/4 ton with a conversion top.
http://en.wikipedia.org/wiki/Chevrolet_C/K - history of the Chevy Truck 1960 to present.
The history of the Suburban -
http://www.classictruckshop.com/clubs/earlyburbs/history.htm
I believe that at one time, I can remember the GMC's having the badge Carryall and the Chevrolets wearing the badge - Suburban. The only difference was the front ends.