Bob,
Hi. Ford offered an optional sliding rear window on pickups in 1935, '36, and '37. May have also been available on later years too. These rear windows where of the split type in which one half would slide open from one side to the other. You'll notice that your pickup should have two window grooves in its opening to accept this option. There was no roll down type window offer on these years.
You may want to contact Lawson Cox who is pretty sharp when it comes to these year pickups. He replies to many questions on this forum. Hope this helps you out. Take care.
Keep V-8ing,
Dale